One because it’s the law and two because the disabled can’t just make a choice and install Chrome.
However, while the site creator does have to meet the disabled halfway, the disabled person is responsible for having whatever type of equipment they need to make it work - ie screenreaders
If your website is full of divs generated by JS that are full of aria tags that make no sense, those tools don't have a chance. Most websites act this way as well. Even Facebook used to lock people out of their messages if you couldn't use a mouse, at least in the last time I checked (infinite feed + no way to skip feed via tab -> can't reach right panel).
Just do your job right. Not saying you should test some unique Firefox config but at least the default version is to be tested.
Hell, I've seen people here indicating that they just tell desktop Mac users to "install Chrome". Such carelessness is bad for business. Web development sure could raise its bar.
However, while the site creator does have to meet the disabled halfway, the disabled person is responsible for having whatever type of equipment they need to make it work - ie screenreaders