I mean, people do this for email. I want email, I heard Gmail is good, so I go to Gmail’s site.

The mere existence of a protocol isn’t the issue here. I think the problem is more that the protocol doesn’t have champions like your ISP/Microsoft/Google. Mastodon saying “come here then go somewhere else” isn’t good; it’d be better if there was a default option somehow.

It’s a common problem with open source; I worked on Passit and we had to think about the problem when it came time to log in to the extension. We ended up saying something along the lines of “check the box if you’re not using app.passit.io”. The default choice matters a lot.

With email, you don't go to email.com, click a sign up link, then get a list of hundreds of email providers to choose from.

Also, the precedent for people looking into Mastodon is not email users looking for a more real time email client. It's Twitter users looking for Twitter on not-Twitter.com

Another key difference with email is that it's easy (the default) to find email providers that are overtly content-neutral, whereas Mastodon, for whatever reason, makes that shockingly difficult.

Just pick one and try, there's no commitment anyway. You can access content from other servers anyway

Is there some way to carry over followers between addresses that I'm unaware of?

Yes: https://docs.joinmastodon.org/user/moving/

How exactly do you do this after the instance admin has banned you?

You don't. If your admin is a total dick and prevents you from migrating, you're screwed. If you have hundreds of followers, better reevaluate your initial choice of instance. If you have thousands of followers, host your own, or pay somebody to host it for you. This however doesn't matter if you're just getting started.

There are talks about detaching user identity from the instance. If this gets traction, this problem might be solved once and for all.

Remember when you were a kid and you did things out of curiosity, to see what would happen? That's the mindset you need in this case. No real harm is done by picking the "wrong" instance.