It's probably not fair: you're doing skilled labor, specifically to put money in your parents' pockets, while earning very little yourself
There are a lot of factors which we don't know about that could make your situation seem worse, or better. This portion of your youth does seem unpleasant and irritating, but that those things are front and center for you doesn't necessarily define the whole situation well.
That's the biggest piece of my answer: while I definitely believe that work deserves compensation, I have also observed that young adults who have not lived on their own often have lopsided views about work, compensation, and free time. None if this is meant to be a criticism of you-- from your question, you sound like a hardworking person that is honestly upset about a real issue. So the following are just things to think about, not indictments of you in any way. And from what you describe, I do think that your parents should be paying you, or at least setting a portion of their earnings aside for you to get later (all else being equal).
1. Life is often unfair, and people that are not legally adults have less agency
Unfair things happen all the time. Even if something is universally acknowledged as unfair, that alone doesn't change anything. Sometimes unfair things can be changed, or avoided. In other cases, they just have to be endured.
2. It can be hard to evaluate work value in an informal setting
Again, this is not pointed at you specifically. But I have known many teenagers who, despite nominally working for a few hours, get very little done in that time-- they aren't necessarily working hard or well. Professional workers also often have extra expenses, requiring them to charge more, which informal workers do not. Examples include taxes, union dues, insurance of various kinds, fees associated with operating a business, etc. All of those need to be paid before the worker can actually earn a profit that they can use to pay their living expenses. You, as an under-the-table worker, have none of those expenses, and your living expenses are covered by your parents.
It could easily be the case that you are being unfairly exploited, but unless you've worked out the math on the cost of maintaining yourself at your current standard of living and the work value of your labor it might be helpful to be open to the idea that you are not (necessarily) getting such a bad deal. Worse than other people your age in your community, maybe, but that's a different matter. And young adults often think of things in... less than objective ways. That doesn't sound like you in this case, but I would always advise an adolescent to reflect on whether or not they, themselves, are being as fair as they could be when unhappy about what they feel they are owed.
It's almost certainly the case that your parents are getting a deal. If having you do the work cost them more money than doing it themselves or hiring out, they wouldn't put you to work. But if, all things considered, you are being cheated of $0.50 per hour for 100 hours' work, that is a very different story from being cheated out of $50,000 in profits resulting from your work on each house. Both are unfair, but one is a bigger deal than the other.
3. Free time is common for young adults, but not a guarantee
Some of your frustrations seem based in the fact that this work cuts into your free time. I can sympathize with that! But "free time" is a tenuous concept for a lot of non-adults. If, for example, your parents didn't demand that you do this construction work but instead insisted that you study for several hours per day, you could find yourself with less free time than you have now and no work that anyone would expect compensation for. If your parents did all the construction work themselves but demanded that you supervise your younger siblings, you might find yourself with no more free time and no more money (even though the alternative might be hiring a nanny, or similar).
Before I had to live on my own, I really, really underestimated how much time and effort it takes to be a functional adult in society, and that same misestimation has been common in others I've spoken with. I had an unrealistic conception of how much free time a typical person can expect out of life, and while I often felt that I was deprived of free time as a child and young adult, I typically have less free time now than I did then, despite being in full control of my own schedule and activities. There have been periods of my adult life when getting a reliable 90 minutes of free time per day seemed like an impossibly sweet dream.
4. Your parents' finances are an important part of the story, about which we know nothing
However they may appear to you, it's possible that your parents have a Scrooge McDuck-style vault filled with riches which they never, ever give you access to out of greed. It's also possible that they are struggling to make ends meet and have little to no money to pay you, regardless of how much you deserve it. House flipping isn't cheap, but it also ties up a lot of money for extended periods, often requires financial leverage, and is subject to some risk. If they simply can't afford to pay you more than they have, or to hire a professional in your place, that doesn't make matters more fair to you but does change the perspective a bit. If your labor has a real chance of being the decisive factor between your family being homed and being homeless (maybe not that likely for you, but a reality for some), the perspective is even more different.
5. Many people resent aspects of their childhoods
That doesn't make anything more fair, of course, but know at least that you aren't alone in not liking elements of how your parents have structured your life to this point. I hated many things about how I was raised, but that changed nothing, and I never saw a dime in compensation for them. That's just how things go sometimes.
6. You're getting a bit more than you probably think, even if it's less than you deserve
This might be a reach, since I don't know much about how you have been thinking about these issues. But practical knowledge about how to build, renovate, and maintain a house is pretty valuable, and many people don't have it. If you want to get a job in a construction trade, even just during the summers, you have a lot of skills that will put you in contention for one. You'll have a more diverse array of tasks you can do for money than many people have, and at a younger age.
It also gives you nice options for your own home someday, should you choose to own one. You don't have to flip houses to get the benefits-- if a renovation you'd really like would cost $90,000 to hire out, but you could do it yourself for $40,000, that's money right back in your pocket. And the capability to do so is only there because of this work in your youth, unfair though it might be. And you might be surprised at how little skill average people have with this sort of work-- you might find lots of opportunities for side work, and the ability to put a bit more cash into your pocket is really nice. Many teenager jobs don't teach any marketable skills you can take with you.
7. Even if you're not getting what you really deserve, you're getting a lot more than nothing
This is more a peace-of-mind, quality-of-life thing, but it may be worth thinking more on the things that you do get rather than the things you don't get but feel you deserve. An Instant Pot is a pretty nice gift. They are nice, versatile appliances, and it's certainly more than the battered saucepan I took to college.
No matter how strong the case is that you should have gotten more, fixating on it is very unlikely to make your life better in any way.