It's trivial to make systemd do that if that is what you want, but there are also plenty of cases when that is not what you want and you then end up trying to write crash-proof startup scripts to provide backoff instead of just changing a flag in a unit file.

(And if you want a dumb unit system, there are plenty of options which will run just fine under systemd as a single unit so you never have to actually use systemd for your own services even if you're forced to use systemd for the overall system for whatever reason)