I think a large part of it is the assumed pre-vetting. The guy in a relationship is assumed to have good qualities, otherwise his girlfriend would have dumped him (and this is judged relative to the hotness of his current girlfriend). A guy without a girlfriend is judged to be a loser, by the same criteria.
One possible way out of this for single guys, is to be seen hanging out with a cute woman.
My dad pointed that out to me when after a girlfriend of mine attended a Crew race and I suddenly noticed a TON of female attention from women on the Crew team. She was hot - so I was obviously desired by hot women - and she was healthy and happy and SAFE, and coming back for more - all good indications.
For single guys I recommend NEVER taking “in a relationship” off your status on Facebook or other sites - and as you recommended - hanging out with a beautiful woman can NEVER hurt.
Cultivate friendships with women single guys - when you are single OR in a relationship. One woman I met on the Crew team many years ago is still a close friend - and besides the benefits of female advice and a hot woman to hang out with where other women can see - she has hooked me up several times with single women she knows.
As much as I would have liked to have “hit that” years ago - maintaining a friendship has paid dividends year after year.