I agree it shouldn't be available just to people like us. My assumptions here are twofold, one Google doesn't want to deal with fallout from users being harmed

I agree it shouldn't be available just to people like us. My assumptions here are twofold, one Google doesn't want to deal with fallout from users being harmed in anyway by an extension installed from their ecosystem. It could have real monetary and regulatory impact on them. Two, this benefits their ad business by reducing how thoroughly ads can be blocked. I think browsers should be moved away from for profit organizations to separate non corporate stewardship. With that you remove the overall susceptibility to fallout and can try to give more freedom over the features a browser has or how deeply extensions can integrate, for everyone. You also obviously remove the immediate monetary incentive to restrict freedoms.