libby they do get snapped up, but they also get put back in the pond because their partner cheats (or they do), they have grown apart, they were just a bad match who ended up having DCs together and trying to make it work etc. Not every man who is single in his 30s/40s is a twat.
When my DP left for work this morning he brought me a cup of tea in bed to wake me up, because I needed to be up in half an hour. He told me yesterday that he adores me and that I am incredibly beautiful (I'm not, but I'm glad he thinks I am!) He gave me my nightly footrub and told me how lucky he is. Then he spent a loooong time in bed making sure I was happy before he was 
This is a man whose ex cheated on him, who got together with her too young and stupid but was trying to make it work for the DCs. Whose ex used to call him a fat cunt, but I think he's gorgeous whatever he weighs.
This is a man who never really cared about making anyone else happy in bed, who WAS selfish in a lot of ways. He is someone who has his faults like anyone, but I love him so much I see past them when someone else couldn't.
And he has learned lessons about give and take since his previous relationship. He has learned the pleasure that you can get from making others happy and he realises that life is happier when you give as well as take. We are able to communicate well with each other and that helps - some people just aren't up for talking.
In my previous marriage I didn't want sex, I was miserable, our goals for family time/free time were mismatched. That didn't mean I was a shit wife - I just wasn't right for him. Similarly someone else who isn't trying to raise children with my ex might find his refreshingly unburdened 
And yes my ex tried the 'unrealistic expectations' crap too - "nobody else would be how I wanted him to be, men just aren't like that" etc. Who would want a fat 40 year old divorcee? My DP as it happens, who loves spending time with me and my DCs, who is everything I was looking for in a man.
Together we are a great match, our previous partners obviously didn't think so! So don't give up hope that they've all gone.
One man's trash is another man's treasure.