Just to clarify a couple of things:
AI managers in OOTP do platoon. They do it a lot. They not only change lineups, but move players around in the lineup due to handedness. They do that a lot too.
If that isn't happening, the most likely explanation is that the AI evaluates your roster very differently than you do. Maybe fielding ability has something to do with it. Maybe the ratings as the AI sees them don't justify platooning, even though you see something different. Maybe key guys insist on being a starter. It's difficult to know. IMO, there isn't much of a difference in setting the lineups yourself vs. having a button you can click which will make the AI set them exactly the way you want. It's micro-managing either way.
Using Oakland has your model could be difficult. Oakland tends to have a lot of very young players who need to be tested and proven as quickly as possible, so that the team can get value from them before shoveling them off elsewhere. Also, Billy Beane shuffles the deck a lot -- Oakland does a lot more transactions than other teams, looking for what's working right now. So, if you want to see these guys get tested quickly, aggressive platooning by handedness is a good way to help do that. I'm not saying that I know this is their reasoning, but it's a pretty strong hypothesis.
If you have minor leagues, you might look at how your minor league AI managers are setting lineups. It's possible that you will see more platooning on those teams.