I always wondered the same thing.

Women from a very young age are fed messages that their breasts are sexual things, the image of breasts is so ingrained as a sexual thing by the time they start having babies that they feel uncomfortable seeing them being used as anything but sexual objects. Normalizing breastfeeding is just now starting to happen in the U.S. Other countries it's a lot more pronounced and accepted.

some female are overly turned on my nipple stimulation and might feel as though feeding their babies might turn them on and feel like a pervert

I agree with @cubanese My friend said she felt too uncomfortable and embarrassed to let the lactation consultant at the hospital after her delivery help her so she would only try when no one was in the room and would stop everytime someone walked in... Which is all the time! I had to bite my tongue.. It's a hospital, they're professionals! Worst excuse ever!