Pure greed, sadly
Depending on the workload, a 2 socket 1.5TB RAM machine can host a huge number of VMs - in my case about 120VMs. Moving from per socket to per vm licensing would blow my backup lic costs out of the water. Fortunately, I have permanent sockets, so will hold on to them as long as possible. All of the alternatives are already on either per core, or per VM licenses.
Funny that HW costs used to be the limiting factor for server deployments. The limiting factor is now license costs. Backups, monitoring, AV, OS, DB, etc are pushing the costs. Each is asking for $5-$50 per month per VM. If you don't have grandfathered socket agreements in place, you can easily end up paying >$1000 year/vm just in license fees.