It really bugs me every Sunday when Chris Kamara goes on and on about this.
Lots of professional teams don't put players on posts. Putting players on posts don't stop goals from corners. You're taking two men out of the middle of the box, giving your opponents a bit of an advantage as they aren't outnumbered by defence, and you're playing everyone onside. It is simply a preference thing.
Having players on posts at corners cost Bayern Munich the champions league - both of Man Utd goals in their famous come back win would have been offside if the players they had put on the post instead were in the middle of the box with the rest of the team.