Because that browser provides a universal API that works across pretty much all the popular operating systems, whereas the alternative is to build and maintain code for each of those systems separately.
There's of course Chromium, but the code base of Chrome is so huge, I'm not even sure Chromium has been properly audited to be "clean" wrt being subservient to Mountain View.
I do understand the old dream of a "universal API", it's snake oil that has been peddled to the IT crowd since I was a teenager (looong time ago).
I also wish it could happen, but Chrome certainly isn't it.