Using free software to create free software is already a good reason.

But if you need more:

• owned by Microsoft, it is a US-based megacorporate product with value to deliver to shareholders first
• never forget EEE as we see a new form of it with Microsoft trying to control the entire developer experience from the server hosting to the editor/AI on folks’ machines; under-litigation Copilot is a straight exploitation of the Commons selling our hardwork back to us
• proprietary means you can’t fork or fix the numerous bugs in the platform nor is there a real issue tracker so you beg on their forums for fixes (anti-free software mentality)
• lock-in issues since aside from specifically the Git part, every one of those proprietary features you buy into will dig a further trench to make it hard to migrate elsewhere
• yes, privacy benefits of not just you but all potential contributors as well since it is a locked ecosystem that requires an account
• not everyone thinks software forges should double as a social media platform with upvotes, FOMO, commit anxiety with employers imploring you have metrics on a closed platform with knock-on issues like star-hacking where projects try to inflate their star numbers in this popularity contest instead of judging projects on merit
• related, the README used to be a file you could read without rendering but now instead they are full of trash markup, emoji, & the repository is filled with binary blobs of images or worse videos for your demo ballooing cloning all wrapped in a Microsoft UI not your own; setting up a separate site isn’t hard (nor is it easy either) but at least you get to own your look & keep assets out of your repository
• there are literally ads & upsells all over the platform
• you can’t use search or see the collapsed comments without authentication
• censorship is not uncommon--especially when it mess with the corporate status quo (see Nintendo Switch emulator dev, youtube-dl, etc.)
• being US-based & big enough for scrutiny, MS GitHub is required to follow US sanctions which prohibit some of your potential users/contributors from even accessing your code (and/or issue tracker and/or forum and/or wiki and/or donations if using MS GitHub)
• …& there already is a host of good alternatives out there for code forges with better performance & features, some of them aren’t locked to Git either; ‘network effect’ be damned