Because you're still promoting the Chrome web monoculture by using the same rendering and JS engines.

I would argue that it is better, but not as much better as Firefox. The further you can get away from Google Chrome the better, but any distance is beneficial.

I think Brave hews closely enough to upstream that even if they leave out a feature like this, they still help enforce Google's control of the web by enforcing Chrome's status as "the standard," that many devs target.

But, otoh, maybe perfect is the enemy of good here.

Also because Brave is pretty sketchy, like promoting their own cryptocurrency, replacing sites' ads with their own ads, etc.