While Spam is a problem and affects decentralized systems more easily (if they have a critical number of users), the cost of client attestation is just too high.
I am perfectly happy if the web and stays open and a lot of people go into the app space and stay there. I am happy for facebook and don't think I am missing out on the web. I don't use any apps for social media and exclusively use browsers. I wouldn't want a second app space on the web at all because the mobile environment is an ugly abomination of software crap.
If we have a form of RA, it will get worse for users and developers alike. It will be a far worse hassle than killing a bit of spam and we give the wrong players too much power.
If you're a 2023-web purist who's willing to just avoid whole services because they're not on your preferred platform, then hw-backed web RA would make no difference to you even if it could be implemented (which IMO it can't): you'd avoid the services that use it just like you already do today.
It is simply the wrong approach to focus on the negative, in this case spam or in general hostile bots.