The browser vendors can earn small commission from each transaction. This ensures steady stream of income for them as well. They have created a piece of public infrastructure that is absolutely critical they should earn from direct payments of their users instead of relying on single entity who may or may not have their best interest at heart.
So if this became popular enough, then in a sense browser makers would be gatekeeping who’s allowed to receive payments. Website owners could of course still show ads or independently earn money other ways, but wouldn’t browser makers then be incentivized to tailor the browser experience to favor sites that earn them a commission?