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