Banks are subject to much stricter regulation than any tech company is or ever will be (IMO, I could end up surprised).
I trust my bank to know who I am and keep track of my money. I trust big tech to lose or misinterpret my data, to close my account for no reason, etc, and to face zero consequences for those failures -- not even fines.
That's true, but remember what we're talking about: Using banks (or another trusted third-party) to assert your age instead of having big tech collecting your personal information themselves. There is context here. Writing comments in a vacuum makes no sense.
In that context it is understood that only incentive for big tech to follow the proposed is regulation, but if you are going to push regulation on them then you can regulate them just as much as banks.
Your broader point that, in the real world, there isn't much political will to push any of that regulation is also no doubt true, but, again, writing comments in a vacuum makes no sense. The hypothetical of big tech adopting a third-party attestation system was already understood to be just that: hypothetical.