Ubuntu did snapshot the bpool; unfortunately, it did a poor job of garbage collecting the snapshots. Meaning that eventually you would have failing kernel updates due to lack of space, and having to manually clean it up.
Since 22.04, zsys (the tool that did the snapshoting) is not installed by default.
It's a lot easier to manage one pool occupying one large partition on all the disks in the pool than it is to manage any number of partitions without pooling.