if they do have an init system at all, it’s usually not systemd but something much simpler, because the environment/task doesn’t need it and there’s a memory and storage benefit to not shipping the extra bulk
Systemd often just straight up doesn't work in containers due to containers having security restrictions that prevent processes in them from accessing certain parts of the Linux kernel. Systemd tries very hard to be a container manager (nspawn, portable services, cgroups controls, and so on), whereas other init systems don't do this, making them work inside containers