I really think the lack of relationships has mostly do to with socioeconomic pressures.
For younger men, who aren't getting straight-married either, they avoid relationships and marriage because they feel they have nothing to bring to the table except debt. They don't see themselves as worthy of relationship unless their lives are perfect, which includes a good income and lack of debt. It is telling that increasingly, it is working class people without college educations (and related debt) who are in relationships/marriages these days.
For older men, it's the previous in some cases. And in other cases where a middle-aged person is doing well, they don't want to share. It's really that simple. I've met many men who crave companionship and intimacy, but are terrified that they will have to share space, time, money - but mostly money - with another. Just a couple generations ago, people paired off by mutual attraction and desire to be together, and worked out the differences with mutual respect and generosity. Now it's all assortative pairing based on SES.
Allow me a couple recent examples. My brother, while in prison for an extended time, was blinded due to a treatable illness that went untreated by prison docs. Then the prison somehow got him released early on "compassionate grounds." Basically they tossed out a blind criminal because they didn't want to deal with him. As a parolee, he had no access to most welfare and rehabilitative services. A lawyer approached him and sued the prison and the state, and he was awarded a large lump sum, welfare benefits, and a monthly stipend for life.
And he got a girlfriend. A 50 year old career criminal who is blind, and he got a girlfriend within months of release from prison.
Another man I know, age 81, is a successful commercial artist, still working some because he loves the work. His wife died of Alzheimer's 2 years ago. He told me soon after her death that he wanted to have a relationship with a man. And within 6 months, he was engaged to be married to a man in his 50s.
The reason both these two got partners is mostly about having money, and being willing to some extent to share it.
8 years ago I met a lottery winner and we dated for a month. He was obsessed that others would only be interested in him for his money. He was also unrealistic because he wanted to enjoy his wealth, travel, restaurants, etc., but he couldn't find anybody to accompany him because he insisted they pay their own way. He needed another single gay man who had enough money to do the things he wanted to do, and who wanted to do those things and pay for them. That's frankly bizarre.