From my observances of two couples who are similar to what you describe; so this opinion being based on "from the outside looking in", I believe it comes down to social stigma and a familiar/habitual co-dependency. The two men in both of these couples' relationships are unhappy, yet they choose to do nothing and also choose to be unhappy. In their opinions, everyone else should change, they aren't the ones who are wrong...
I bought the 'I'm so unhappy' story after 6 mos of professional involvement with SO . I knew better than to get involved with a married man...but he was sooo convincing. For 5 yrs he lived with me and the divorce dragged on. Then I got cancer. Later I found out that he must have been telling the same story to 4 (that I knew of) women he was seeing while I was back and forth to the hospital and in treatment not to mention he was probably seeing other women when out of town...traveling salesman.
I recovered, he was history...He did marry one of those 4 women ( and she was cheating on her husband - saying they hadn't had sex in years) and I overheard him lying to her about his whereabouts one day...he was with me trying to 'get into my pants'...which didn't happen! Hmm...wonder how things are with them now 15 years later!!
I'd bet dollars to donuts that smooth talking, good looking devil still uses that line...if he has an opportunity!
But to answer your question - Some men just complain because they have no backbone to do the work it requires to fix it. tell them to get counseling and to remember WHY they married in the first place and tell them to act like they did back then and maybe things would change. And advise to you.. don't become a party to the complaints...tell them to seek out a professional even if they have to go alone. We can't and shouldn't get involved mentally or physically!! Discussing such matters brings a feeling of closeness and intimacy in sharing feelings and secrets...don't do it!
Simple answer...because they know they'll get screwed in the courts.
They'll lose the house, she'll claim "I'm afraid" and then he'll be kicked out and will still have to pay for the mortgage. They'll be paying for her life and will quickly find out their quality of life will fall through the floor.
They can't afford to leave so they complain but it's all they can do with todays divorce judgements.
Yet SOMEHOW, the average standard of living for a woman FALLS after a divorce, while the average standard of living for a man RISES after a divorce.
Exceptions abound, but USUALLY the picture you paint is false.