it goes against every fiber of ethics of foss dev's.

Firefox is developed by a nonprofit backed by two corporations (Mozilla Corporation, and MZLA Technologies Corporation), and yet most people seem to support the browser as being ethical. So I wouldn't say corporations are inherently the problem.

it doesn't make sense because foss is there for privacy, security reasons and why would you host it all on one of the biggest and greediest corp of all time?

Who says it's exclusively there for privacy and security? I personally use FOSS because it provides me with an alternative that's affordable (like Libreoffice), or is something that I perceive as being superior to everything else on the market (Home Assistant, Nextcloud, and Mastodon personally are a few).

Privacy and security are still important to me, but I mainly consider my threat model to be different compared to others, so I have a little more trust in companies like Microsoft around certain places. GitHub is also used by some large businesses for hosting their proprietary codebases, so I would say that there is some expectation of security. Granted, we have to take their word for it. But when intellectual property is on the line, I strongly doubt that they're bluffing.

Not everyone, like you, feels comfortable using GitHub though, which is fine. That's why other systems like GitLab and Gitea were built. The core of FOSS, in my opinion, has always been around choice, collaboration, and just sheer will. Linux initially began as a project to get a system running on a single machine. And look where it is now.

And also, contrary to popular belief, free software isn't always inherently secure. It just tends to have vulnerabilities disclosed and patched faster than commercial software only changeable by a small handful.

If you are building foss may as well take a little more effort and put it anywhere else maybe it'll be cheaper too but definetly better in privacy,

There really isn't much of a difference in cost between some of the other major collaborative versions controls, given they all usually have generous free tiers that should suit most moderately sized projects.