Because he can do better.
The fact that he doesn't, indicates that he is a chubby chaser, is with you out of pity or is very bad with women.
Strangely enough, what you are describing happens to men, except for us, it's awesome.
Long boring story time;
I once dated a 10 out of 10 girl, she was tall, long blonde hair with honey colored eyes, with a large, but not too large rack and overall curvy body on a thin frame(MMMmmm), she had a very pretty face as well. Anyway, every single time I was out with her, everyone treated me differently, guys' expressions were like "Yeah, GJ BRO" and occasionally we would exchange nods of approval. Other women would look at me and smile, tossing me looks of "Hmmm, interesting, maybe later , wink wink". It got to the point where I would start getting strange benefits for free, like for example, I would get the best parking available to where ever I would go, which I would normally never be given and other little things like that. I have never experienced that since. I have dated many women, but never one as physically attractive as that one. She was studying to become a dentist as well, so we had -that- in common. Other than that, we had absolutely nothing to talk about.
It didn't last long, but for what it was, I enjoyed it greatly on many levels. To think that she lives like that 24/7, women have such an extremely easy life.