Because kicking for the corner with a decent chase means trapping the receiver in the corner for the first tackle and then having limited options to pass for the second tackle.
If you just kick it out you concede possession to the middle, and their forwards can just amble onside for the restart. That's fine if you're looking to wind down the clock, but shit if you're trying to build pressure.
Of course it's all about the chase, as long as the kick is half decent. So the question shouldn't be why kick to the corners, it should be why is our chase so inconsistent.