Like footballers they get what someone is prepared to pay. It is absurd, but the amount of money in the key teams is absurd too.
In one regard it would be awesome if we were back in the 60s where a top driver would flit between series rather than just sticking to F1, doing it for the pleasure of racing - the money seemed a long way down the list of priorities back then. Heck we even had one weekend when there were two events which counted for world championship points, an ocean apart.
Sadly the second last time a driver did it Kubica nearly paid the price with his life. It was so refreshing to see Hulkenberg attempt the 24hrs, let alone go on and win the thing.