They're not. I just did a search and didn't turn up any of the books I was looking for. Not a single one.
For example, do a search for Stephen King; everything in his catalog is available for normal purchase, but not in the "Unlimited" library. Doesn't work for me.
Then again, Netflix doesn't have very good streaming library either. Anyway, if yo have reasons to complain, Kindle Unlimited clearly isn't geared toward you. It isn't for me either, as I read about 10 books (exclude reference and education) per year.