Two reasons:
* For better or worse, humans are not machines. We can only keep doing the same thing over and over for so long before we go crazy, somewhat like Chaplin in Modern Times [1]. Creative work is somewhat less tedious than assembly line work, but there's still a limit.
* Because "non-free" time is time you spend making somebody else happy: Your boss, customers, friends or family, or society at large, e.g living up to expectations that you ought to be successful. Somewhere in there you need to make room to make yourself happy as well, it's not possible to live a life merely by attempting to live up to other people's expectations.
> I take it as implicit that the one thing the elite players are not doing in their "free time" is playing music or that would obviously be a form of practicing.
That doesn't follow. The kind of practice that makes you better is the boring stuff - playing scales and working on your technique. Doing fun stuff like playing your favorite tunes may be part of the training, but it's not the kind of training that improves your skills.
To keep learning, you need to keep pushing yourself into new unfamiliar territory, and to stay focused on mastering it while you're there. And we generally can't stay that focused for more than a couple of hours at a time.