I do not think you will find most people buy their own, in my rather larger family I am the only one who has built a computer, others buy a pre-built as they do not care about being able to save a few quid if that for a basic machine and want one to just work.
Why are MS phones bad? MS do not make any phones, their make the OS, Nokia are the major manufacturer of them, granted WinPhone 7/7.8 were a bit dodgy Phone 8 is a brilliant OS, and if it was so crap why are so many companies swapping to them? Why is o2 now actively pushing the Windows Phone devices?
MS have only 3% of the mobile market android has 73%. That in it's self should tell you how bad the os\phone is.
Why is O2 pushing win mobile? you would have to ask them.