Except that with e-mail there are obvious good reasons, which basically boil down to whether you've centered your digital life around the Google, Microsoft, or Apple ecosystems (or never moved away from Yahoo, etc.).
These are recognizably proven brands that aren't going to disappear next year, and which integrate with your other tools in ways that clearly provide benefit.
And back in the day before webmail was a thing, your ISP gave you an e-mail address so you didn't pick at all.
But none of these are even remotely applicable to Mastodon instances. So I'm not sure that e-mail is actually a helpful analogy at all here.
Maybe not from the outside. But after a few months of actively using the platform with a curious mindset, I'd wager you'd have a much stronger idea of how instances differ. That's the main service I see the "pseudo-default" mastodon.social providing in the current world—as a "jumping off" point that people can use prior to migrating to more specific instances with more customizations or better communities.
That's what I meant by "no catch-all choices yet". There is no reason Google or Apple couldn't get in the game if they choose to. Supposedly Facebook is making a go at it for whatever that's worth.
I offered the email analogy because that helped me to wrap my head around it when I was starting out.
Email is much better than that. Even if you don't want to maintain your own email server you can still register your own domain and assign it to a hosting service for a trivial amount.
You can also do that with Mastodon if you're keen, so email analogy holds.
These are recognizably proven brands that aren't going to disappear next year, and which integrate with your other tools in ways that clearly provide benefit.
And back in the day before webmail was a thing, your ISP gave you an e-mail address so you didn't pick at all.
But none of these are even remotely applicable to Mastodon instances. So I'm not sure that e-mail is actually a helpful analogy at all here.