Someone could decide to forward their other mail to their fastmail account. Should they then potentially risk email their other customers send to that address? DMARC headers tries to solve this, but the world is dirty, mailing list software suck, and their they would have to take the blame for problems outside their control.
I can understand the decision. They could probably do something to show good intentions, like flagging suspicious email and making sure their own email software shows appropriate warnings, but it's never going to be perfect.
SPF, DKIM and DMARC do not provide authentication of non-envelope headers like From: and To: etc, unless they are specifically included, but there is no way to publish that you require those headers as part of the DKIM signature.
Stopping phishing is hard. End users mostly are fooled by a little padlock in their web browser, and that's a much simpler trust model. Eliminating email dressed up as web pages would probably do more to combat that than authenticated sender models ever will, but nobody really wants that.
"Email spoofing bugs do not qualify. We are quite aware that users can set arbitrary From addresses on emails, that our SPF records allow arbitrary hosts to send email as our domains, and that our DMARC policy is not enforcing passes. These policy decisions are by design, and we track the actual sender in a separate header."
[1] https://www.fastmail.com/about/bugbounty.html