Couple of things (having lived in this world). The people running this stuff aren't by default stupid (well, not always), and not assuming they are helps with understanding.
First, someone has to pay to migrate apps, and as often as not the resources to do that aren't available. Sometimes you do the math and it doesn't make finacial sense to allocate those resources. And sometimes you just can't move apps. The vendor might not support vm/cloud/web deployment. Or, very common in manufacturing, you have an app that runs a machine and requires a very, very specific hw/sw environment to be supported.
Next, there's that whole regulated thing. We spend a lot of time in front of federal/national regulators explaining how we do things so that we don't screw up your mortgage or whatever. And those regulators take careful notes. So if I've spent the last 10 years explaining how we have this amazing VMWare deployment that ticks all the boxes, they're going to want some very, very detailed explanations of how HP VME or Proxmox or whatever is going to tick all the same boxes at least as good as VMWare did. And we're inviting a lot of scrutiny for a couple of years to make sure we did it right. It's a lot of work.
Sometimes you weigh everything, not just 'it costs less' or 'Broadcom are a bunch of assholes' or 'we don't want bros on HN to think we're suckers', and the answer is legitimately "the divorce isn't worth it".