The issue is that there is a lot more to a home server than updates and backups, and even doing those correctly automatically is... pretty hard if you don't have very strong opinions about how apps need to support them.
So to give you an idea, Sandstorm requires: An app always must recover from forced termination (the only way it shuts apps down!) and that a given app carry all of the code it needs to update its data from any previous version. If you don't do these things, your homelabber will inevitably be fixing stuff themselves in various apps because their server lost power or the app developer demanded some bespoke intermediate inane update steps, or you missed a few versions while your server was off.
What percentage of apps made available via Docker meets those requirements? Probably a lot less than you'd think. (Most of them that do easily work on Sandstorm!) But if you want to make selfhosting work for non-sysadmins (or sysadmins who don't want to do a second job when they get home from their main job), you have to do these things.
Another thing is that you basically have to have security right because your users and even your server admin probably aren't experts, and definitely aren't paying for an enterprise-class firewall. So a big part of Sandstorm's opinionated app design is around making it fundamentally impossible for app vulnerabilities to exist. Things like proxying everything through the platform, isolating individual documents into their own app instances, handling authentication and authorization, etc. all become core to making apps secure enough to not have to worry about them that much.
This is especially important because a lot of selfhosted apps get abandoned and unmaintained, but selfhosters still want to use them. Of course, all these things fundamentally require changes to the app your random Docker container shipper can't insist on, and that's before we get even into performance concerns, which are key since selfhosters tend to use old or cheap hardware.