Openstack on k8s is basically giving up on openstack on openstack. https://wiki.openstack.org/wiki/TripleO You need some kind of orchestration for the control plane. Either you create it from scratch or use something existing.
I am totally unsurprised people gave up on Triple-O.
I haven't personally worked on OpenStack setups myself, but my coworkers in few places did (including supporting if not actually being contracted out to build some of the commercial offerings in that space) and especially upgrades were always this huge project where it made more sense to tear down the entire setup and bring it up again.
That was made easier with OpenStack packaged as docker containers, but ansible was arguably still more pain to setup the cluster than just using k8s.
TL;DR setting up OpenStack was so horrible I think SAP started deploying it through k8s.
So if you want to setup local "private cloud" kind of setup it makes sense to set up OpenStack on k8s.
If you then want to provide multiple clusters cloud-style to the rest of the organization... well, it's just layered again.
In fact, at least one significantly-sized european vendor in on-prem k8s space did exactly that kind of sandwich, to my knowledge.