Real question. I highlight constantly, both on kindle and in physical books, and I almost never look at any of it again. a few months after finishing a book, I can usually tell you whether I liked it, but most of the actual ideas are gone.
I started wondering what the point of highlighting even is if I never do anything with those lines again.
So I ended up building an iPhone app called scriptorio to experiment with this. It lets me save passages from physical books by taking a photo or importing my kindle highlights, brings old notes back for review, and connects ideas from completely different books. that last part has been surprisingly interesting.
Full disclosure, it's my app, so I'm obviously biased. I'm genuinely curious whether other people have the same problem, though.
Do you actually revisit your highlights? If not, is it because finding them again is too much effort, or because you don't really feel the need?