They [don't] do it, because they have a business model. Same goes for Google. The problem for google is that apparently this other tool is already available, today, though the website is currently overloaded so I can't reach it.
But if that site starts working for me, later today, why would I ever ask Google anything again?
That's not a good analogy. There are architectural reasons why AP/fediverse will never work, no matter how hard anyone tries. It is not business reasons that prevent, say, Facebook from adopting ActivityPub. They are prevented from adopting it by critical thinking.
Rubbish, lad. These platforms manage accounts in their millions within the garden every day, and you're telling me that they can't manage to open up a hole in the API to let someone add and read posts that way, rather than through their sadistic web interfaces? After everything they've already done?
More to the point, ActivityPub is just the current popular prototype, the Bitcoin if you will, of the federated social space. We'll get it sorted just fine.
I think a Google Search Premium that cost $50/month would go over pretty poorly with Google's existing customer base (advertisers), but a startup can experiment with the business model with the right early adopters (e.g. Neeva).