Again, you're kind of giving the game away about knowing not a lot about F1. It isn't that the drivers weren't "that great" – in F1 it is the whole team that competes and wins. The team, from management down, were not the unit that they became when Schumacher arrived and an incredible effort was built around him.
And in any case, what you've pointed out is the vast difference between Schumacher and those other drivers.
For you to imagine for a second that he might have gone to Ferrari and won a Championship straight away (because the previous drivers "weren't that great") does, again, make you look a bit naive. Sorry, not having a dig, but it does.
By the way, Ferrari also failed to win Championships with Mansell and Prost.