That's actually a design decision made to work around the limitation of having to run as root. Since the container had to be started as a privileged user, if you want non-privileged users to be able to start contianers you either need a setuid binary or a client/server interface like docker has. There are pros and cons to each of those options.

Now it's possible to create containers without elevated privileges (assuming user namespaces are allowed), but that wasn't possible until long after docker had established the client/server model.