Because unless it's really necessary for compatibility reasons (which is most often true only in a corporate business setting) a product like Microsoft Office can replaced by a free alternative that's just as good. So if you really don't need to spend the money, why should you? I've used MS Office for Mac before, and I can say I much prefer NeoOffice. I'm glad it's around, because I found MS Office for Mac slow to load, laggy, and as bug-ridden as any Microsoft product. Any open-source program will generally be of very high quality, and--though not necessarily as feature-ridden--more stable than the commercial product, because the source code is there for millions of programmers across the globe to scan through and assure its doing what its supposed to do the best it can. I'm a university student, and NeoOffice has met and surpassed my needs. I have high hopes for OpenOffice.org's native Mac release when it's ready, as well as Lotus Symphony. I have no need for MS Office again, and until iWork offers me a lot more for the dollar, I'll stick with my free, open-source alternatives.
If I were in a corporate setting where complete compatibility was a necessity, then my needs would be different, but being able save as .doc is really all the compatibility I need right now. So don't chalk it up to being cheap by saying "kids these days..." Honestly, it's not like illegally downloading music or movies; we're talking about quality, open-source applications that offer just as much as the commercial product (for most people's needs) and just happen to be free. What's the problem? Should I spend the $100 anyway?