To a large extent, it's the "trophy boyfriend" issue. They want somebody who their circle of (female) friends will be impressed with. Dating a plumber would lower their status, and they fear being gossiped about and laughed at behind their backs. My friend (in construction) found several women who felt he was good to hang out with, as long as the relationship was kept "discrete".
There's a similar dynamic with male executives, just not as bad.
truth bump
You've never heard of the "trophy wife"?
Too true. Whenever I found out that a woman I was interested in was a higher-up in some company I knew that as a simple Technician she was not going to be interested in dating someone beneath her -and it was ALWAYS the case, lol. The tables turned when I became an Engineer, though, as suddenly the receptionists, assembly girls and clerks all suddenly seemed to be extremely interested in getting to know Me much better. *grin* I could tell you some quite humerous war stories concerning that. Almost got caught a few times and can laugh about it now. I even started noticeing some speculative gazes from some of the female Engineers and one or two of the Executive staff women. I had learned My lesson by then, however, and was unwilling to be a chess piece in their corporate maneuvres.