[daneel_w]

You're missing the complexity of making the guest inform the host that it has fully freed this and that slab of memory and that the host may reclaim it until further notice. It's a bit more complicated than the other way around, where the guest believes it has e.g. 4 GiB of RAM available but the host doesn't allocate all of it for the guest until it tries to read/write there. A virtual machine is something entirely else than a containerized piece of software.