IRS has to work with my tax attorney. Banks have to insure money (up to a point). Banks also have to be careful about shenanigans. If people lose confidence, they would be a run on the banks, which they prefer to avoid.

Google/FB have to ... well, nothing. I have the same say with them as I would with a video game company and in-game gold. They could decide I simply don't exist. Ticket closed. Block chain (from the implementations I have seen) does not solve this.