That's bc AK is a better hand than 77. It's "behind" PPs, but it's still better against the possible range of hands your opponent might be playing. It dominates any ace or king your opponent is playing, and is at worst a coinflip against a pair. 77 can be in far more trouble preflop, and is at best a coinflip. Besides, it's a trickier hand to play postflop after a raise.

EDIT: I also raise 77 here.