Three primary reasons:
1. My college-age son wanted it badly enough to promise annual vacations w/me to WDW...since I'd been anticipating him not wanting to vacation w/Mom for too much longer, that was huge--(no, I won't "hold him to it" and yes, I know "life happens" and he might not be able to...but going into each year "planning" on a WDW trip (over T-giving, our family tradition) is a big emotional factor for me...
2. It's prepaid good living space with quite a lot of choice about where...no matter how broke I might be next year, or the year after, or five years from now, I've got "housing" (GOOD housing) at WDW...all I have to do is come up w/transportation (and mostly frequent flier miles will take care of that)...at the
DVC villa, I can eat as I would at home if I'm really broke, and I buy APs, so after each year's purchase, park admissions are prepaid, too...so, at my resort and at my level of points, I get all this for about $50/month in dues...
3. I own a (very tiny, miniscule) piece of the magic...that makes me feel good in a way that's contributed to my happiness at having bought into DVC...