Why GitHub?

For one: microsoft hosts a bunch of OSS on GitHub for free, where these projects use all sorts of resources (storage, compute, etc).
They get this for free is because a lot of OSS is not funded. So there's not even a point in asking those projects for money because they simply don't have it. But these resources cost github money, regardless.
It was beneficial for github to offer this 'free' service to OSS because it facilitated their tipping point, their clout, their user base, it turned github into a social network. This is step 1 of the enshittification: make life nice for your users.

But when OSS projects get funding through this ESTF, then microsoft can go to those projects and state "you have money now, you should pay for your github usage ...or else...". This threat would not be a vacuous one, because while a project could easily move its code (git clone && git remote add && git push new-remote), moving its operations, its issues, its community, its CI/CD, is a much, much, much harder task. These projects will suddenly realize that github turned a decentralized source code control system, into a centralized prison.
Given how thinly stretched many OSS projects are, I would venture to guess that they will probably deprioritize moving away from github and pay to get rid of the nagging for money by github's owner.

I don't think this is about funding OSS. I think this is about microsoft finding new ways to get other people's money. They didn't even bother with step 2 of enshittification, and went straight to step 3 ("your money is mine"), albeit in a veiled manner.

Call me cynical... but then I've been around, so there's that...

All that being said, I think a ESTF as such is a good idea; maintainers and other contributors deserve to be compensated for their efforts. They do a massive amount of thankless work. The unintended consequence of this will - I fear - probably be that maintainers will continue to be uncompensated for their efforts because the money will go to github (for those hosted there) and whichever other what-will-be-a-previously-free code hosting service instead of to the maintainers and contributors.