If you ask any IT team lead, the real reason is the usability and it-just-works qualities of the software.
If you ask most IT team leads, the real reason is that they know that users in general treat computers like voodoo - perform a particular ritual a particular way, and you get the desired outcome. This lack of mental flexibility means that when someone learns a particular GUI they are not keen to change to a new one - which is the reason you get exactly the same inertia about switching to a new version of MS Office (vis: all that Ribbon hoo-hah) that you do for switching to another OS (with it's other applications with other GUIs).
This is the "usability" part of that statement. That's the reason that people railed so heaviliy against Windows 8. Why do you think MS invest so heavily in giving copies of their software to schools? Get those GUI rituals in peoples heads.
As for it-just-works... MS software does plenty of infuriating and irritating does-not-just-work things.
* Linux : I can move a file while I have it open in an editor, and saving the file in the editor saves to the new location
* Windows : Won't let you move the file
Microsoft would solely have to lean on selling support and consultation services after that.
I can imagine that terrifies them ; presently, even if you pay for support, you get very little. You get better support for Windows and other MS software from the community. With popular OSS projects, you typically get good support from both the community and the authors, AND you get the ability to look at the source code to understand your problem better or even fix it (or hire a contractor to do this). This is one of the cornerstones of why I use OSS wherever possible in my technology stack - the larger the software company gets, the less my problems matter to them. IBM manages just fine in this model.
Windows works today, out of the box.
This is so untrue on so many levels.
When I install Linux, it usually takes about 20 minutes, with no driver downloads (because I do my homework and buy compatible hardware). Most distro's leave you with a machine that has a bunch of useful applications, out of the box.
With Windows, I've had to hunt for drivers, download drivers, slipstream special drivers into special install disk images (so that the install can proceed far enough for the real drivers to be installed...). This is for machines that were sold with Windows and provided with install images. It literally took me all night to reinstall my wife's laptop (reboot! reboot! reboot!) after her office decided that because the Linux install didn't support their proprietary disk encryption program it wasn't suitable (never mind that it had perfectly good encryption on it anyway). And that's just for the core OS, never mind the vast list of applications that you have to add to make it even marginally useful.
At that moment, the Linux guy will still be applying various fancy patches and trying out different distro and desktop environment combinations to see which works best.
I use Linux for all my real, productive work on a daily basis, use stock packages for the vast majority of things, use the standard Ubuntu image, again, out of the box, without doing anything to it bar installing packages and configuring a few of the options a little.
Unlike Windows, I don't need to tweak my install ; If I move to another machine (say, a hardware replacement cycle), I can literally move the disk from one machine to another and keep on trucking - Windows throws the most epic tantrum imaginable if you try that. If I want to go crazy and upgrade to a new version of the OS, I back up my home folder, install the new OS, install the packages I had before with a single command, restore my home folder and move over most of my files and config folders... and I'm off again. Again, if you try that on Windows, you're screwed, because most of the apps store all manner of settings and license keys in the registry, which you can't just remove from one machine and transplant to another.
Enterprise setups? Both require work. The amount of screwing around with Active Directory that it takes to maintain our machines at work is legendary.