Because dude loves this wrestling shyt. I don't even think money is a major motivation, but he can make major bank for his fam outside of WWE anyway, with a light schedule
Plus the only doctor who says he shouldn't be wrestling is the one on WWE's payroll.
I'm not even saying he should be wrestling. There's definitely some warning signs, and he'd be an idiot to not least switch up his working style, but the reality is he'll probably be trading headbutts with Ishii and Suzuki within months of coming back.4
But at the same time there's plenty of guys on the roster who are currently wrestling who have had their brains scrambled as much if not worse than Bryan.