Yeah well we're just the plebs and we use whatever big tech forces us to use and whatever they allow us to discover through their discovery platforms and recommendation engines. About your Fastmail example specifically; All non-approved email services are blocked; if you try to set up your own service, all your outbound emails on behalf of your customers are marked as spam by big tech. Seriously, try to setup your own SMTP email service.

Every service has a moat; sometimes a technical one (I.e. block competitors as spammers), sometimes the moat is regulations (Are you SOC2 compliant?); other times it's just a business contract between people who belong to the same club.