When I was much younger I used to try to put the drivers all clearly on a ladder in different series from best to worse. Surely F1 drivers were the best, then those open wheelers in the States, then the dudes driving the tanks in NASCAR... Then I learned that was silly and not how things worked. Juvenile Canadian patriotism played a role in my anti-American racing sentiments, as I'd clearly like to envision European drivers as being better than American ones, or ones racing in American series... Maybe unless a Canadian was good in an American series. Remember, this was when I was a kid.
Racing has many disciplines, oval racing is but one of them. I would not expect an F1 star to go kill it in IndyCar, nor the reverse, for many reasons. It is not 1965 anymore when it was easier to be faster in a wider variety of cars. Things are specialized to laser precision now. That isn't bad or good, it just is.
Why shouldn't F1 race on ovals? Let me count the ways...
The drivers going into F1 are all bred on karts, then junior open wheel formulae in Europe. There are no ovals for them to race on, and the sport has no (the Indy 500 does not count in my eyes) heritage on ovals. If not being able to race on an oval means F1 drivers or cars are not the best in the world, then I think you have a myopic view of racing.
Yes, IC guys are more versatile, running on a variety of ovals, road courses, and streets but this doesn't matter in my eyes.
The majority of the fanbase of F1 do not care about ovals, seeing as they are not North American.
The added design specifications required to make these cars run on ovals would add expense and would likely slow the cars down on other courses, so even if you ran them on a couple of ovals the cost and added design would be high.
The tires would need to be designed for both oval and road course running, which almost assuredly would lead to added cost and compromised design if the same kind of tire was used on both oval and road (we don't want another 2005 USGP).
There are probably many more reasons, but these came into my head easily.
If versatility is your criteria for greatness, then I suppose you are right. As I said, I now just see every racing series as its own unique thing. I think comparisons are never that easy to make.The inclusion of ovals is one of the reasons why I considered CART of the 1980s and early 90s to be perhaps a greater series than F1. They showed a level of versatility in ability that F1 cars simply lacked.
Then again, if your criteria for greatest was most advanced cars, biggest budgets for teams and drivers, and the fastest cars on road courses, F1 would be considered "greater" or "better" than CART in the 80s and early 90s.
The one thing I will say is that I think F1 drivers are absolutely the fastest (does this mean best?) open wheel road and street racers in the world, as they race in the cars that are the fastest and most advanced on these tracks, and they are paid the most (capitalism works!). That is a comparison with other series I am willing to make.
F1 does not explicitly claim to be the best though they do everything but say it. They certainly don't have to run on ovals to prove to anyone or themselves that their drivers/cars are the best, whatever that means.