Because senior management forces workers to use it.
I'm a scientist who analyzes large data sets. I also need to communicate to my co-workers. I need a secure operating system without a lot of eye-candy that makes it look like a tablet and lets me give priority to my data analysis tasks. Windows is not it, but I have to use it.
I just don't get how we still allow closed source operating systems for critical business tasks.
If you were to poll corporate user income and their provided OS, you will likely find that 99% of workers earning low wages (i.e. not trusted by their companies) are provided windows, the more someone earns the more likely they are to have a choice in OS from their employer--just my hypothesis.
Maybe I'm a bit behind the times, but some collection of VM, MVS, OS/390 aka zOS, CICS, IMS, DB2, RACF, SNA, Oracle, SAP etc etc is probably running the majority of America's critical business tasks. Did it all get open sourced when I wasn't looking?
Needless to say, Google Docs and Gmail are not open source either....