The why part is easy. Stupid people know Windows. They drag and drop files onto it and it copies. Job done.
Any IT department can ghost an image onto a computer. Job done.
Clever people know that a small amount of prep time - getting a working linux image and a cron job to fetch video packs - will save them a lot of headache in the long run, but Linux, or NetBSD, or FreeBSD, or OpenBSD, or any of the others are all too niche. Too unknown for non-technical people.
While the entry requirement on knowledge is falling - and the average user (as opposed to developer or enthusiast) intelligence of the open source software world is dropping as a result, the usability hasn't proceeded at the same rate and the intelligence needed for average companies is still too high.