"They get the idea eventually but in my experience it takes several years." Same. My mum was one of the worst and she's spent nearly 50 years in an abusive marriage!
I think sometimes it's jealousy/envy (see above). Sometimes it's a narrow world view, sometimes it's ridiculous fear that you'll go after THEIR dh/dp!
Single 15 years here, have casually dated on occasion, had fwb arrangements when they suited, but no interest in a full on relationship.
I like doing what I like, when I like, having nobody to answer to, and the bed to myself at night!
There's pros and cons to both singledom & being in a relationship but also people have different priorities.
I've always been an independent type, something every man I've been in a relationship with has had issue with. Women less so but depends on the woman. Some are independent types like me, a few were on clingy side. Just different people.
If I could find someone who was happy to occasionally do something like go for dinner, or to a show, have sex sometimes but not want to "progress" the relationship to moving in etc that might suit me, but generally I'm happy staying single.
It's very tempting to turn round to the ones that are vocal but in shit relationships themselves and say "why the hell would I want to be in a relationship like yours?! And be abused/ignored/cheated on/taken for granted/whatever?"