Impact. At a smaller but rapidly growing company, you could end up personally implementing or leading a significant chunk of the product. At FAANG you are likely to be working on a small, not very exciting corner of something.

Focus. One of the FAANG culture diseases is to do a lot of engineering work of dubious to negative value. Doing NIH versions of popular open source projects or migrating and replacing stuff that works fine with stuff that works at best a little worse. When engineering time is more scarce, you’re more likely to be doing something important.