How do you hone your skills by winning every race? That means that you're far superior to everyone out there, and no matter what you do, you win. Meaning you don't learn from your mistakes. And that is pretty much the opposite of what any racer, from 5 to Pro, wants to do. We learn through our failures, not the things we do correctly. By pushing hard, making an error, coming back the next weekend and making better on an opportunity.

And just a personal anecdote here:
In the 4/5s (I was never actually a 5) I never came close to winning a race. I simply didn't have the sprint for it, and the tactical side at that level is pretty much nonexistent. I made my upgrade to the threes through accumulation of top 10s. In the 3s, I've racked up more podiums and top 10 finishes than I did in the 4s. I was forced to figure out how I was going to survive, and how I was going to be able to be a factor, other than a field sprint. In the 4s it was just a matter of making sure I closed gaps and put myself in a decent position most of the time, which doesn't fly in the 3s. So, I took my strength, and worked on it, and worked on it, and... you get the point. Coupled with a newly found tactical knowledge and my ability to start, bridge to, and work in breaks, I'm able to get the results.

Soon enough, I'll have to figure all of this out again, and get used to life in the Pro/1/2 field (later this month...). And, that probably means getting a coach. My rate of recovery is still improving, but my fitness is just slowly creeping upward.