Hate to be the one to defend Google here, but the reasons weren't that unreasonable. I want my browser to prevent random extensions from directly reading web page data. The declarative API idea is pretty good.
It's just that uBlock Origin is so insanely useful, important and trusted, it should get full access to the entire browser regardless. Honestly, it should be literally built into the browser instead of being a mere extension. Only the conflicts of interest inherent in an ad company maintaining an ad blocker prevent that.