Why is Data so dumb?

Data has an information storage capacity of one hundred petabytes, computational speed of sixty trillion operations per second, is capable of creative thinking himself and understanding any "original" metaphors of others (e.g. when Dr. Crusher likens the away team to mosquitos in The Best of Both Worlds--rewatched this just now, which is what brought this topic up), but we're supposed to believe somehow Data doesn't know what idioms are and/or at least a few examples of them? (e.g. "Early bird catches the worm."--Best of Both Worlds pt. 1)

I really don't mind because it has a narrative purpose--like how a certain teenager's allowed to pilot a starship--but for a franchise that oftentimes goes out of its way to work out how things work in-universe, you'd think the writers would have stopped resorting to this after the first season or so.

*edit: link formatting