Most infant guidance is based on a baby's age in weeks or months. From my very limited experience of parenting (7 months), it seems to me that it would make more sense to base any information and guidance on the weight and developmental milestones of the baby.
For instance, shouldn't guidance for sleep and food requirements for a 25 percentile 7 month old that crawls and is very active be different to that for a larger 7 month old which isn't yet very physically active?
Practicality.
Not every baby is going to be the same, and guidelines for one baby are never going to be perfect for another baby with the same attribute, regardless of what you decide to base it off of.
Age tends to be used as a baseline because it's the only abstract thing that all babies do equally - age.
Say for your example you have two babies who are exactly the same weight, but one is much older than the other. Clearly they are not equal, especially as far as nutrition is concerned (perhaps one baby is overweight, or perhaps one is malnourished.)
This is why guidelines should be treated as such: Guidelines, not exact rules. You will need to use some common sense and self-judgement to determine if your child is at the appropriate age/weight/maturity/etc to follow that guideline.
Essentially, calendar age is a much simpler metric than weight and physical activity. It therefore is a simpler guideline for parents to follow, particularly if your child is out of infancy and going for longer stretches between pediatric checkups. If I am curious how much sleep a ten-month-old should get and I want to research the question, I easily know "ten months old," but finding an accurate weight can be more challenging and physical activity can be a very subjective measure.
It is also important to note that this simplicity is adequate for most children. If a significant enough deviation from average is noticed, more nuanced guidance is definitely available. For example, we were simply told our baby should eat and sleep as much she wanted. I first heard anything about specific caloric requirements when she was three years old, because she had begun to fall "off the curve" and the pediatrician wanted us to track her intake more closely. We weren't told "toddlers require n calories," but rather got advice based on a combination of height (average) and weight (< 2%ile).
