Weil du so viel Lehre machst. Mehr noch als ein FH-Prof, von denen auch kaum einer noch ernsthaft forscht.The Gymnasiallehrer I know have a lot of free time. Maybe not as much as you'd think from an outside perspective, but once you settle into a routine and stop caring too much, it's a decent gig.
I'd assume though, with the incentive to do serious research removed, you can no longer be bothered. Apart from the fact that you're also no longer exposed to research as much via seminars etc., even though in principle you could watch some talks online.
Fair enough. I guess one stays motivated only if s/he really keeps on wanting to return to the academia. But being surrounded by grateful kids is very rewarding, so one soon sees that no way you would want to get back to that grumpy self-centred and corrupted world