They don't have to rebuild the intersection; they just have to redo the signal timing so that the thru green for the bike lane isn't in the same phase as the right turn green for the general purpose lane. (Okay, so they might have to add a right arrow signal head too. Still, cheaper than new pavement.)

They just don't want to do it because it would probably reduce level of service for cars and they don't care enough about cyclists' safety to make that trade-off.