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.
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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.