If you are said parent, which of the kids do you absolutely trust to never ever screw you over no matter how much incentive there is and no matter how hard a times they fall on?
That's what I'm worried about when getting old(er). You give your trust to an amazing person in their 40's with the best of lives. They'll take care of you. Then they fall on hard times 10 years later and oh hey look they have power of attorney over all this money maybe if they help themselves juuuuuust a little bit ...
When I was 10 my dad drained my and my sister's savings[1] account that way. Just a little, just to get over the hump in his business, he'll put it all back later of course. 20 years later and afaik he still hasn't.
Having access to other people's money when you have none of your own is terrible. I don't know if I'd trust myself with that, let alone somebody else.
[1] of course it wasn't really our savings, think of it more like a college fund my parents started for each kid when they were born