That's probably where the borrowing ends. They start fresh for every Smash game.
Just take the characters for example. The returning characters are completely remade each time. Attacks, sounds, animations etc. Then you think about how they interact with the new characters, are there any clear advantages/disadvantages, how they handle on stages, items and so on.
That's why balancing the roster takes such a long time to get right. There are far too many factors to consider, if they simply built on top of the previous game hardly anything will change. Each game would feel almost the same, and we clearly know that isn't the case.