Different reasons, I think.

As an 'enabler' of an alcoholic mother, I can give insight there. In the case of enabling - for me - it's because of an innate desire to feel needed, and loved. I'm lonely elsewhere, and I fear that if I go against her desires (in other words, allow her to face the consequences of her alcoholism or encourage her to recover, and stop allowing it) that she will reject me. I realize, deep down, that it's better to have her be angry at me for a little while, than to keep enabling her addiction, but my fear of rejection still makes it hard for me to stop enabling her. If that makes sense?

As far as people tolerating jerks, I did read once that those people just tend to be more... confident? I'm not sure if that's the right word. But more social, in any case. So people have to interact with them more. I'm not entirely convinced they're always liked. They're just impossible to ignore/avoid, and most people don't want to be rude themselves (even if the person is rude). This can give the illusion of popularity (or being well liked).

As far as the kid question, it is very rude. I never ask it, and I don't think anyone should. Ever. There are many reasons people don't have kids (one being they can't). A lot of it just goes back to social constructs. We are raised, pretty much, to believe having children is the end goal. Those of us that can't, don't, or don't want to (I'm the latter) are thus often questioned about it. These normal - seemingly harmless - questions bother me, too. Such as 'What do you do?'. Saying nothing always gets an awkward "... Oh."