The fact that Linux may be cheap or free does not stop large companies having a business model that allows them to make some extremely good revenue out of the applications that run on that O/S. Go ask IBM about Linux on System z running under z/VM.
IBM makes a lot more money on its own proprietary operating systems. So do other companies. When they support or offer Linux, it's mainly to respond to a demand by IT managers who don't know any better than to insist on whatever they last read about in the trade rags.
The proprietary operating systems are often dramatically superior to Linux for a given type of job, too. Mainframe operating systems, for example, are extremely productive for the types of work for which they are designed, far more so than a generic OS like Linux. Even UNIX is a terribly poor choice for mainframes, and if misguided customers didn't insist on it, it wouldn't be in the catalog.