There are some women who legitimately don't try and get attached as they go through their 20s, and when they hit their early 30s and start to think about having a family and settling down, they end up doing okay.
But the remaining 9% of women who are 30 and single have multiple red flags. If it's not anything obvious (face, weight), it's in there somewhere. There are two girls I was really into, well before I met my wife, that seemed like the perfect package....fun, cool, very cute, great body, good job, etc. At the time, it sucked that I couldn't get them, but as I sit here now and they are both 32 and still single despite very much wanting to be in a relationship for YEARS, there's something going on well beneath the surface that has guys running away and running quickly.
BTW, that doesn't mean that the unmarried 30+ men don't have red flags as well. We're all pretty ####ed up, married or not. We just hide things and get past things a lot better.