God damn I thought I saw the last of these stupid browser takes on reddit, chromium is open source and we've seen multiple browsers (Brave, Vivaldi, I think librewolf) using its potential to remove themselves from the chromium baseline and build out their own fork with ad blocking services that didn't go down when manifest V3 happened.
There's no "browser monopoly" anymore than there's a "V shaped engine" monopoly in cars. Why don't people use Gecko more? That's like asking why people don't use rotary piston engines in cars, you could, it's just garbage. Gecko isn't the standard because no one wants to build a web browser with it.