Yeah I read a blog post about why it isn't and the answer was pretty bog-standard answers for why anything is closed-source: "if we make these cool customizations open, then anyone could take it and make a competing product."
https://vivaldi.com/blog/technology/why-isnt-vivaldi-browser-open-source/
It's because there's effectively a class-action suit going on right now, but because the user agreement says you have to use arbitration, there were tens of thousands of people who are like "sure let's go to arbitration". Valve is losing tons of money having to fight all the suits.
https://www.classaction.org/steam-antitrust-refund-2023
Note: I am one of the people involved in this suit.