Well, so far I haven’t seen anyone giving a compelling reason why they should still exist.
The best explanation that they’re cheaper than printing on the cover ignores the fact that thousands of books of all kinds (children’s books, reference books, even the occasional coffee table-type epic book) don’t have them, and are often very cheap to buy new.
Cardboard covers are also printed on both sides, so that eliminates the inside flap argument. In fact, some of the books I own have both a full cover dust jacket, and full colour printing on the cardboard cover.
Covers can also be glossy and so have equal brightness and vivid colour levels to paper dust jackets.
It seems to be a legacy that’s being tenaciously clung to. Perhaps it’s time for publishers to step back and rethink.