Because people will buy it, that's how.
Nobody forces anyone to buy Windows software, or update their Windows software. Hell, up until recently, Microsoft was still supporting XP, an OS from 8 years ago, and they still may, I'm not sure when they were going to drop support for it.
Windows is still partially supporting XP through the availability of downgrading from Vista to Windows XP, don't know if it's possible to downgrade from Windows 7 although.
And well there are somewhat of a case of "you wanna play this, you upgrade!" in ways of the old Xbox not supporting the newer games, and if you send one of the old Xbox's to repair you'll get a Xbox 360 in exchange if it's supported under their warranty as far as I've read.
and even Vista was generally disdained as a poor operating system, even though for most home users it was okay. On a side note, and I've never really noticed this before, it seems like every other OS has been bad...so beware the next version of Windows.
I've never seen the hate for Windows Vista, it was a decent well secured operative system, although you could argue "oh holy fuck it was too damn secure!!" sure, the pop-ups eventually could drive you crazy, I managed to live with it although seeing as the only pop-ups I have a problem with is advertisement pop-ups.
I have to admit while it may annoy people, I loved the security measurements Microsoft took for Vista, I read that they tested the UAC (User Account Control for those who don't know, that would be the pop-ups) with Symantec launching about 2000 malwares, worms, trojans, rootkits etc. in general description, a lot of viruses, and supposingly the UAC blocked 50% of every single threat.
I'd say that's enough for me to not mind the pop-ups, and again, I don't mind Vista, I liked it, a lot.
I will say that I would definitely trust Windows far more than I'd trust a Mac.
Sure, but Mac also has it's fair advantages at points, especially with the whole damn lock-down of operative system codes on laptops (while I know it has a majority to blame on the manufacturers of the laptop, *cough* HP!! *cough*) which is a troublesome thing to deal with, also something that has annoyed me with Vista and Windows 7 is the problem to adjust the harddisk to a new computer if the chipset or whatever it is that's usually causing the BSOD if you launch a pre-installed HDD from 1 laptop / stationary to another laptop / stationary.
Sure you could praise the fact that it's easy to update drivers as opposed to XP, but XP at least is adjustable to another laptop, and as far as I know (never tested it properly) Mac operative systems are also.