However taking responsibility of your own funds is very different than taking responsibility for someone else's. The stakes are much higher, especially if the amounts are non-trivial.

In other words I trust myself with my own self-custody, but I wouldn't ever want to be in a position where I can lose someone else's savings (people close to me that trust my judgement, to make things worse). Maybe the group of potential Uncle Jims is much smaller than the group of people who know how to self custody.