It's simple, cost. Supporting multiple DE's is expensive. And provides little or no benefit to the company.
It may work at a small company with tech savvy users (like the ones commenting here). But ultimately at a normal large business, is nothing but a hassle that at best makes a few employees happy.
I work for a large company that issues Windows laptops or MacBooks to employees depending on the work requirements. Most developers I know there use Macs, and I've only heard of 1-2 cases where programmers needed to get a Windows machine because they were working on a particular project.
Those few employees are probably going to all be developers, and despite there being a bunch of mathematics and engineering involved, being a developer is very much a creative process. Similarly, I wouldn't begrudge a digital artist for wanting to use a Mac to do their work.
If a developer is asking for a thing, they're not asking for it because they've suddenly developed a nervous tic. There's typically a reason behind it. Maybe its because they want to learn that thing to stay relevant, or explore it's feasibility, or maybe it's to support another project.
I used to get the old "we don't support thing because nobody uses thing" a lot. The problem with that thinking is that unless support for whatever thing immaculates out of nowhere it'll just never happen. And that's a tough sell for a developer who needs to stay relevant.
I remember in like 2019 I asked for my company to host git repos on the corporate network, and I got a hard no. Same line, there wasn't a need, nobody uses git. I was astounded. I thought my request was pretty benign and would just sail right through because by that point it was almost an industry standard to use git. I vented about it to some devs in another department and learned that they had a system with local admin attached to the corporate network that somehow IT didn't know about. They were using that to host their repos.
I guess what I'm trying to say is that if keeping employees happy is too expensive, then you gotta at least be aware of the potential costs of unhappy employees.
My last employer had several thousand employees. Some of the IT guys knew Linux, but it wasn't anywhere in the organization. I managed to convince them to let me install Linux on my desktop. They said sure, with the provision that I was not allowed to have a single issue. If I had an issue, they'd format it back. It was a fantastic last 8-9 years at work, as far as computer use went.
My usual reply to said employees is "if you know how to install and configure a Linux distro, you probably also know how to solve your own problems". Everything else is pretty much deployed over AD, so if you can get to the point where you need admin creds to hook to the DCs, then do whatever you like.
Eventually, all of them failed to even get close to being a part of the AD DC and that is where the story ended.
learned that they had a system with local admin attached to the corporate network that somehow IT didn't know about. They were using that to host their repos.
That's called shadow IT and is a huge security risk.