ZFS root absolutely is supported on Ubuntu and Debian. Every Debian system I've built in the past year or so have been 100% ZFS (around 6 physical systems, but I've also built AWS AMIs this way). You have to install via debootstrap, but it's definitely a working and supported configuration. Installer support would sure be welcome though, it's not trivial!
And that’s exactly the issue. You can’t just use ubiquity to set it up, or even the terminal, you have to go a long, complicated, badly documented path.