Some nipples require less effort than nursing from the breast. Milk flows from some just by holding them upside down. In this case, the child only has to stop the flow by covering the opening with the tongue to rest or to swallow. Some children prefer this pattern to the sucking necessary for the breast.
Of course, the strength for later eating and speech is developed during the feeding process. As a speech language pathologist that works with children with feeding and speech problems, I would encourage careful selection of nipples and pacifiers. These devices should be carefully chosen to promote oral motor strength within the child's functional abilities.
The Avent Naturally Feeding Bottle is available in a variety of flow rates. It is an excellent nipple for oral motor development. It often works well for breastfed babies and they have soft spout cups that work well for transitioning to cup drinking later. I'm not sure what flow rate nipple you are presently using, but you might try going to a slower flow rate if possible to make your baby work harder since the hypothesis is that bottle feeding is easier than the breast.
You might research different nipples and find one that more closely matches the breast. You might try the Evenflo Classic Sensitive Response (silicone). This nipple works the cheeks and lips and is not for a baby with weak musculature. It does flow during pauses in sucking however.
I encourage parents to compare nipples and pacifiers by trying them. With careful experimentation, you can identify the subtle differences in effort, pattern, etc.
Of course, she may rebel against another nipple at first, but it may be worth the aggravation of changing.