Given that half the games are anime waifu sims these days, I can only imagine what horrors you'd unleash letting people link to their profiles on anime waifu mastodon instances.
Actually no, I don't have to imagine it since I've seen the horrors with my own eyes.
Then why don't they accept any instance? Fediseer has an api where they can see the most reputable instances and limit to only allowing those reputable instances.
Because of what everyone else here is saying. If you're a company like Valve, you don't want to open a portal to undesirable content that isn't moderated. They chose the biggest instances with the most users because those are generally going to be the safest ones.