I think that comment is a bit naive.
Where do you think F1 drivers come from?
They don't just "hop in", they have to work their way up through the ranks of motorsport.
Many F1 drivers have competed in many different types of motorsport.
Mark Webber, Michael Schumacher, David Coulthard, Johnny Herbert, Alan McNish and many others have all raced in sucessfully in sports cars and at Le Mans, for instance.
The reason that no F1 drivers compete in other motorsports is because they hae exclusive contracts.
Michael Schumacher earned £40 million odd last year, and is worth a lot more to Ferrari if he wins the WDC and MC, and he has value for his sponsors as well.
If he were to break his leg racing on a "weekend off", that would be devastating for Ferrari's F1 campaign. So they don't take the risk, and there are probably a lot of things about that in his contract.