I think the man-child has become sort of a fantasy protagonist in our pop culture. There are different flavors of man-child characters, but the common thread is that they’re well-liked by everyone around them, and comfortably successful in spite of how immature and inept they are. The narrative is written from their point of view, so you can see that they’re trying and they mean well.
Homer Simpson is probably the ultimate example (and Peter Griffin, who is very similar.) Michael from the Office is a different type; he fails in his personal life, but no matter how bumbling and inept he is, he is still the boss. A lot of comedy protagonists are like this, like many Adam Sandler characters. Seth Rogen’s character in Knocked Up is a pretty good example. At the beginning, he’s a pot-smoking, unemployed slacker, but he gets the girl in the end. (Most characters like this get the girl.)
If society had an “ideal” man, a common thread is that this type of character falls short of that. They’re often lazy, sloppy, drunk/ high, or just a little dim-witted, but they mean well. The other common thread is that they have a lot of the hallmarks of success you’d expect the “ideal” man to have- nice house, comfortable lifestyle, loving friends/ family, hot girlfriend/ wife, etc- and the viewing audience can see that they’re really nice, well-meaning people.
* I kind of think that male characters who “don’t play by society’s rules” deserve an entry here. Like, Han Solo isn’t sloppy or dim-witted, but he does seem to be part of this spectrum of characters. *
Most of our media is made by men, so this narrative must be really appealing to men these days. Family Guy or the Simpsons are escapist fun because they say that you don’t really have to try, and things will work out.
I don’t think this narrative comes from TV and movies. TV and movies are an expression of underlying trends in our culture. Maybe the modern world is becoming too much for men, and they want to escape their responsibilities? The wives/ girlfriends of the characters in these examples are usually more capable and competent, and they sort of take care of the men, but they also seem sort of nagging. Maybe this negative, nagging side of the female characters is another fantasy element. They’re responsible, but it’s portrayed negatively, so in the end, it says to the viewer that you don’t have to be responsible.
This is probably also why romance novels are so popular with women- there’s the fantasy of being swept off your feet by someone who’s strong and capable. Romance novel heroines are often bland audience surrogates (like Bella in Twilight.) They don’t really do anything to get the guy (or do anything, period.)
I don’t really know where I’m going with this, so… end comment.