Only because we let them. We gave them web domination, we the nerds told everyone and their mother to install Chrome so they do out of habit now.
We sit and watch while Google bend the web to their will with AMP and dozens of non standard specs everyone hurriedly tries to support while we come up with excuses like "it faster", "the dev tools in chrome are better".
Without a monopolistic marketshsre we give them they can't do this as easily.
Go install Firefox and make it your default browser. Or just carry on using Chrome and wonder why Google keep getting away with doing antisocial, anti consumer shit.
This works because they have such a large market share. It's a chicken/egg problem. Switching to Firefox is the best most of us can do to achieve the goal of "getting people to stop following Google's leadership on all things web."
Yes, supporting other browsers is a given, but did you want to say "and advocate"? Advocating for alternatives alone will achieve close to nothing when you are faced with a cartel, regulatory action is desperately needed in this industry.
Or advocate for open alternatives, use Firefox, and make sure any site you build (or you build at work) work perfectly in non Chrome browsers.