Cart and the IRL have their own problems with one another, not to add F1 to their list of competition.
By your logic, only the talent meant for ovals is used by those ovals...the rest is left untapped (as in: "Oval drivers cannot drive roadrace cars").
The REAL problem is the financial acuity of CART and the IRL to NOT attract top drivers, or even entice parnets to start their children driving young...where's the money in that?
Can we all agree, that in the US, the money is in selfish Baseball, Football,Golf, and Basketball players? With even a percentage of those resources thrown at racing, we'd have a great pool of drivers to choose from. However, as it is, junior is more apt to grow up and become a football star than a CART star.
It's in our culture, and you are correct in saying that NASCAR is closer to football at times than it is to CART....