Small email providers are doomed, though. If they don't have an address from their ISP, just about everybody just gets a GMail account, or ProtonMail if they're more privacy focused.
I'd like to host my own mail but it's not worth it anymore thanks to spammers.
Are they? I'm seeing self-hosted Postfix as the most common mail server in scans. For free, personal email hosting I agree but that's just a fraction of the market.
For very general reasons small things outnumber big things. In the case of services this tends to mean that most users are using one of a very small set of providers despite the many smaller providers outnumbering the few big ones.
I'd like to host my own mail but it's not worth it anymore thanks to spammers.