By and large, games don't run on Linux (except some of the best games ever, like Nethack, and Dwarf Fortress ), so there's a $90+ cost to get your foot in the door. You're going to spend hundreds on a GPU and CPU, probably hundreds on Steam--but do spend what you can at GoG, even though their library is relatively tiny, because they're awesome--and $90 at build time makes you cringe? I lucked up and didn't have to buy a new OS, but I figured my hardware budget with the cost of a new Pro OEM license already removed from it, in case my activations had been used up.
Likewise, if dealing with people that want/need Windows, or want/need Office, it costs what it costs, and not having it costs a lot more than the software.
As far as builds go, many people will fall into some set of the following, without any pirating:
1) They are students, or work at a college, and get it free or discounted; or have some other means, like a work and home program, MSDN, Action Pack, etc., to get it cheap; or have already paid for several licenses through such systems, as part of getting access to other important software licenses. FI, some schools have deals where if you need software for courses, you pay a flat fee and get like 90% of it, usually including a current-version OS license--the software makers want you hooked on their stuff when you leave school, after all! Some companies will also make deals where every employee gets a free or discounted option, so as not to be using old insecure incompatible software at home on account of being cheap. Not everyone has these opportunities, but if you work for government agencies, schools, or IT companies, they often have some way of allowing you to get Windows, and other software, cheaper.
2) Due it being a fixed cost, they have already removed it from the budget before they started looking at what they could spend on hardware.
3) They bought it when it went on sale somewhere, in preparation for a future upgrade, knowing the regular price was likely to be $90+ when it came time for parts. Many did this with the 7 family packs, and the early Win 8 deals. Windows 7 still goes on sale for anywhere from $60 to $80, so if you knew you'd be upgrading or building some other new Windows boxes, buying one or more copies when it was one sale would make more sense than waiting for when you needed to buy hardware, as long as you knew you'd use it.