One of the main advantages of OSX is it just works, increasing productivity in many fields. Apple supplys both the hardware and software and makes sure they work together well. If they made OSX for any x86 platform, not only would they kill their hardware buisness as Sunner correctly states, but OSX/mac experience would lose some of it's key advantages. I've built quite a few PCs, and windows does have it's strong points (more software and gaming compatability). That being said, I switched over to using a powerbook from a home built PC about a year and a half ago, and I will tell you that the powerbook, with OSX, is much more reliable from my point of view. There hasn't been one thing yet that it hasn't been able to do, save play the latest and greatest games. From a student's point of view, that's okay, as I have an x-box to play games on, and I can still play WoW enjoyably on my powerbook.
I also like the fact that I don't have to worry about getting viruses daily. In the year that I used my PC on campus (the one that's in my sig), it was virtually problem free. That is, until it got a virus a week before finals week - a headache to say the least. I had been using the corporate edition of symantic (sp?) antivirus provided by my school, and had been running updates daily and doing fully HDD sweeps at night, every night, and it still happened. The thing is, most of the people here are power users who are more computer savy than the normal PC user. You all are smart enough that windows works for you, and in many cases works well. However, in my own oppinion, and take that for what it's worth it's just an oppinion, OSX on a mac is better for the average joe user who doesn't need to run any speciality software designed only for windows. This is a HUGE HUGE HUGE portion of the personal computing market. Many people won't even try a mac because they are afraid of change (and I'll admit I was never a mac fan until I tried one at a computer lab). However, they won't hesitate to call me up to get rid of their popups, spyware, viruses, reformat their dell for the third time, etc... To anyone who just needs their computer for the average stuff (IE: internet access, watching movies, presentations, music, word processing, web design, minor graphics work, etc...) I find it hard not to recomend a mac over a PC. Now if you're a hardware buff, gamer, etc... there's nothing at all wrong with a windows PC, as long as you know what you're doing.