LXC does have a container manager though, which is a single process that stays alive for the life of a single container. Within runc (the runtime Docker uses), we don't have a container manager but the downside is that now the upper level needs to keep alive the descriptors and other kernel objects that allow for safe container management by the runtime.
[I maintain runc, and collaborate with the LXC folks.]
[I maintain runc, and collaborate with the LXC folks.]