It's not just computability, networks are involved. If this was all on one machine, you're right, there's no reason you couldn't change it, but there's delays and information losses and even bad actors involved. To deal with this, ActivityPub assumes users are confined to servers at the very core of it's concept.
I'm not an attorney, but I know enough about the law - or at least policy - to attempt an analogy. It's like a legislator trying to add a requirement that all corporations are fiduciaries to their clients. Fiduciaries exist, and work. They couldn't work like that, though, because a market economy assumes a certain amount of pursuit of self-interest. How the hell would a corporation handle all the conflicts of interest that would arise? What happens when they inevitably misunderstand what the interest of a client is? What about when there's multiple parties fulfilling different functions in the same project, but who may have competing interests?
You could try and make a non-market system where production is handled by fiduciaries, and you could even call it "capitalism 2.0", but it wouldn't really be capitalism anymore - that would be the blockchain thing. I don't know what the Pixelfed approach would be equivalent to, but it's basically mimicking the functionality of the feature (fiduciary duties) without actually implementing them. Maybe just really strong consumer protection regulations.
To be clear, market principles are ActivityPub here. If I'm imposing funny ideas about law, no offense, but we're even. The takeaway is that decentralisation makes things a lot more nuanced than they ever are on a machine you fully control.