I think the same reason some people find Google's advertising-based business model problematic, they find Brave's advertising-based business model problematic.
Not that I'm advocating for ads that track and profile you and everything that you do. We should be making an effort to create an infrastructure for ads where they can't track and profile us. That's what Brave is trying to do, and I don't understand why people are against it. Like, how do they expect sites to be financed?
Lots of people on HN make money with ad networks or are otherwise invested in the concept of harvesting user data.
Same thing happens with GDPR.