Because sometimes it is easier to ship one thing to customers than 10 different images and several different run manifests.
There's plenty of other reasons.
given that containers usually do not include any init system at all that’s not a good reason to pick a side.
Ubuntu Docker images do not run systemd by default or in most configurations. In many images it is removed entirely.
I wouldn't be too concerned about that in a container since you're probably not running systemd in that context.
As far as I can tell, the recommended way to run several processes in an Ubuntu container is under supervisord. The default Ubuntu containers don't even include an init system.
Suoervisord is an awful thing to do under a container tbf. Any oom event and it will kill a random process. Not good in a container environment.
I've only ever done it when I needed to run exactly two processes- porting an older piece of software that was not originally designed to run inside containers, and orchestrating the separate processes to be able to communicate and run in separate containers didn't seem like worth the effort.