The main reason is tech debt and proprietary software. Most companies have decades of software infrastructure all built on Microsoft based systems. Transitioning all that stuff to Linux is a massive investment, especially taking into account the downtime it'll cause combined with the temporary decrease in productivity when everyone has to get trained and build up experience with the new platform.
And then you have to deal with proprietary software. A lot of niche corporate or industrial hardware only supports Windows. And you probably have to regularly interact with customers who use Windows and share files with you that can only be opened in Windows only proprietary software.
Linux also frequently struggles with a lot of weird driver issues and other weird quirks, causing an increased burden on the IT department.
Basically you're looking at a massive investment in the short term, for significantly reduced productivity in the long run. And all that mostly to save a bit of hardware costs, which are only a fraction of the operating costs for most companies. Just sticking with Windows ends up being the more economical choice for most companies.
I am typing this on a 5 year old Android phone. It has 128GB of memory and 8GB of RAM, very decent cameras, a beautiful OLED screen and a processor that is more than fast enough for everything I do with it. And even now the battery still lasts two days with normal use. It cost me about €300 at the time.
Unfortunately the Android version is getting so far behind that some apps are starting to get a few issues, so I have been checking out some black Friday deals for new phones, but they look very disappointing.
In the current market it seems like I'd have to pay about €500 to effectively just get a side-grade. All €300 offerings look like just a straight up downgrade in any way apart from the more recent android version.
So I think I'll hold on to this one a while longer. Hardware-wise it's still in perfect condition, and if software support really becomes an issue then perhaps I'll try out a custom ROM.