Because they (incorrectly) think that most men are looking for the stereotypical 18-24 year old, size 8, who has perky breasts, a tight tummy, and a firm/pert but just-a-handful bottom. It's all about the confidence (thanks Gok!) and knowing how to market yourself. If a fat(ter) lady thinks she's competing with the size 8s of the adultwork world, she will do all she can to hide the fact she ISN'T size 8, and this will actually do the complete opposite of what she's trying to achieve: hooking punters who WANT to punt with her. She will lose business by trying to compete with or pretend she's something that she quite clearly isn't.
So if she grew a pair (ahem) and actually EMBRACED her size, shape, weight, and wrote her profile accordingly, she would actually attract the clients she seeks: clients who WANT a fat(ter) curvy(er) heavy(er) wobbly(er) encounter, while at the same time not putting off the clients who are seeking a size 10 and finding a size 14. This would also ensure a higher return rate, as the guys who DO see her won't be disappointed (about her body/size/weight) and will be more likely to become a repeat visitor.
("she" in the above instance is ANY lady who tried to pretend she's slimmer than she really is, and tries to compete with ladies within that size bracket.)