It's only too lean if they're WILD. If you raise your own, you can fatten 'em up a bit.
And who would try to live on venison, anyway? You'd have a milch cow, that would throw git (calves) that you would need to take away from the cow in order to get the milk,
and the calves are VEAL ~ tenderest, tastiest, etc.
Same with goats ~ under 35 days old, a kid is considered Cabrito ~ sweet and tasty eating.