It's a web application too, not just a SMTP server. The MailHog web app is a little rough around the edges (it's an early AngularJS app) and had some bugs involving HTML mail content, so I can see the need for an updated version.
I do the same with mailcatcher but by self-host I mean I would throw it in the projects docker compose file for local development and spin it up specifically for that project and take it down when not needed.
> Mailpit was originally inspired by MailHog which is no longer maintained and hasn't seen active development or security updates for a few years now.