Because you can't necessarily run everything in Kubernetes, or in the same cluster. OpenStack probably provides VMs, private networks and bunch of other stuff to run legacy systems, 3rd. party software, Windows application, tons of stuff that can't be containerized.
You can have a large Kubernetes cluster running OpenStack, because it's probably the easiest way to deploy and maintain OpenStack. You then build smaller, isolated Kubernetes clusters on top of OpenStack, using VMs.
It's not as crazy as it sounds, but it does feel a little unnecessarily complex.
My money's on Conway's law. There's a hardware team that's in charge of the hardware, and they need to orchestrate ask the nodes, then there's the openstack team that's their customer who’s in charge of providing a cloud-like environment to the rest of the company including windows VMs, then there’s an applications kube team that provides kube for services that run on kube, with finally kube-ized applications teams that run on the very top.
One reason could be that you use Kubernetes as a deployment tool, but you don't actually need to full capacity of three bare metal servers. So you need to slice up the physical servers in some way, and Kubernetes can't do that.
From experience most Kubernetes clusters are actually large enough, in terms of capacity required, to justify using an entire modern server and companies are very reluctant to run a mix if various application on the same cluster. There are very very few organisations large enough to need bare metal servers as Kubernetes worker nodes. Unless you use them to run OpenStack.
You can have a large Kubernetes cluster running OpenStack, because it's probably the easiest way to deploy and maintain OpenStack. You then build smaller, isolated Kubernetes clusters on top of OpenStack, using VMs.
It's not as crazy as it sounds, but it does feel a little unnecessarily complex.