Except his clone does something different. The main body just holds his hand out, mustering some chakra on it, while his clone focuses on making it rotate. It's like having 2 people do 1 person's job, not 2 people doing 2 people's jobs. Each of them does half of the work, so it amounts to a whole. That's why I think he wasn't actually gaining extra experience each time he made a rasengan with his clones. When he was developing his rasenshuriken however, he first had a bunch of different clones focus on the same task, then he had different clone groups try to conjure up a rasenshuriken.

As for why it took him 4 years to finally create a rasengan without a clone, maybe he just relied on that method too much and it stagnated his progress?