There is competition. It’s just that many people don’t know or haven’t tried them. Here are three providers on par with Fastmail but are way cheaper if you need multiple mailboxes — Posteo (posteo.de), Mailbox (mailbox.org) and Runbox (runbox.com).

But I do believe that even these cheaper ones are expensive for what they provide in terms of storage capacity, number of aliases, etc. Costs are supposed to go down over time, and prices too.