Because Google controls the Chromium source.

And when they control the vast majority of browser share (already true):

They add non-standard features, some websites use these features which locks out browsers that follow the standards.

Sure, you could maintain a Chromium fork that strips all the "bad" stuff. But that's a lot of dev time and money.... and it only gets worse with time as they add more. And why go through all the trouble to make your user's experience worse?

And now Google de-facto controls web development standards.

The more users we can get off Chromium the better. Right now it's literally just Firefox and Safari that are holding out.