Do you think major cloud providers haven't engineered the whole "You’ll spend months just finding and configuring a disjointed hodgepodge of tools to get what you could have setup in a day." step?
And virt-manager comes across from their docs as a bit on the basic side, proxmox and ovirt look a bit more polished - but again, companies are often wary of running their entire infrastructure on something where there isn't someone they can blame if it goes wrong - it's more about managing the risk than anything else.
I would absolutely love it if these sorts of companies used and relied on open source technologies more, but it's unlikely that they would pay for the staff with the relevant skills to manage it all, or pay enough to retain those staff that get trained up internally. (And I'm somewhat hopeful that this broadcom/vmware mess will cause more resources to flow into open source projects, or spark new developments in this space - cloud isn't always the answer to everything).
Major cloud providers have huge teams developing their compute as a service, that is what they do, that is the bread and butter.
I don't have the resources and team size to hack our own hypervisor/on-prem cloud solution.
I have a team of 3 people, that is it. I can't put 5 people on designing and maintaining a hodgepodge solution of various tools that get us to where vmware already has us. I need a turn-key solution that a small team can manage.
