Virtually all of Mozilla's income comes from the browser (via the Google search agreement). The vast majority of Google's revenue comes from ad revenue on search, YouTube, and Adsense. Not from Chrome directly. So they had less incentive to reward its security, but did so anyway. And they also do some of the best work in the industry, free, for competitors via Project Zero.
https://textslashplain.com/2024/10/13/content-blocking-in-ma... shows a ten-line ad blocker that blocks Google's ads, https://github.com/extesy/hoverzoom/discussions/670 is a list of polite email messages from people who'd like to have elevated access to browsers.
uBlock Origin Lite blocks YouTube ads just fine.
Do you really think Google wouldn't do anything about as blockers? Especially now that no ads is one of the selling points of YouTube Premium?
And it doesn't matter what I think about it. I'm giving you facts not opinions.
Personally I believe that the browser is intended to defend against e.g. Facebook's apps. Google wants to make sure that if you buy a new device and it comes with a Facebook app preinstalled, it also comes with a browser. And that the browser isn't controlled by anyone who'd like to disrupt any of Google's many nice income streams.