What you are seeing is the product of the great cost to run a team (keeps out independents and start-ups) and the simple supply-demand curve chart that you see in every economics class.

There is a demand for exactly 43 teams to race every week. It took a while but the market has sorted itself out to supply exactly 43 teams every week. No more (because it is too costly to field a team and not make the field) and no less (because if you make the field every week you can turn a profit and keep it going).