They are, just not for desktops. Know why? Because windows is what most people know and training both users and IT staff to use Linux is harder than just going with Windows.
Incorrect. Companies use OSX (my company being a prime example) and they don't care that new staff need training. But Linux doesn't enter into it.
Name six that use OSX as their only OS. (not including Apple)
No massive company will have 100% Apple the same as no company will have 100% PC.
But below are 6 prolific Mac using corps.
Cisco -now have over 35,000 macs.
CBS
Pixar
Google
Viacom
IBM
As for companies that aren't huge, millions of companies use macs - anyone developing apps and phone software, games companies, graphic design companies ...
Give me 6 multi-national firms that issue Linux to employees as the predominant desktop platform.