Probably because they're urethane. Drill it tame, polish it up, and on today's oil conditions it's practically as straight as plastic. You can play them effectively in the dry area, and some bowler's consider them as "harder hitting" than a pancake core plastic ball, and prefer to use them as a "dual purpose" spare ball / dry lane ball.
I'm using an original Faball Black Hammer (polished) in the same role in my arsenal.