All those tools use a Linux VM (whether managed by Qemu or VZ) to run the actual containers, though, which comes with significant overhead. Native support for running containers -- with no need for a VM -- would be huge.
It's more manageable to have automation (i.e., Atlantis) do the apply on the open PR and then only merge if it succeeds. You have to rely on the locking feature of Atlantis to avoid conflicts, but it's worked pretty well at scale at Enova (~50 Terraform changes a day).