EDIT: if you don't know anything about baseball this headline will be unfortunately incomprehensible.
Pride is not an adequate answer for me, because the more bunts you drop against the shift, the better your stats are and the more money you make. This should be clear.
I suspect the deeper reason is that a lot of sluggers simply can't bunt well enough to make this work, and don't want to spend time over the offseason trying to learn. When would someone like David Ortiz learn to bunt? From 8 years old he could hit the ball over whatever fence he was presented with.