Thats because of restrictions that a 26 man roster imposes. Cant have a platoon at every position, and really considering you need to carry 2 catchers, that leaves 3 positions max for platoons. One is prob a defensive guy, so that leaves 1-2.
I agree with you, most teams dont platoon, but I dont think its something that one should go out of their way to avoid. Sometimes, with positional versatility, you can carry a couple more (for example, if you have someoen who can back up a ton of positions, you can carry more platoon oriented bench)
A lot of righties have splits too. I have splits. Im a righty and Ive never ever ever been able to hit a lefty. Well, not well.