I believe it's because the children who manage society and business want their workers and citizens to take their bullshit seriously. So they shaped a false and convenient image of what it means to be an "adult", such as putting work above family and personal life.
Yes, it's because of incentives. In dating, it's seen as a big negative to be a "manchild" or be dating a manchild; in less inflammatory terms, being seen as immature is seen as negative in the dating world. This is actually quite fair: it's natural for someone in a relationship to prefer a responsible person with self-control (e.g. holds a good job, cleans up, and is financially responsible).
A person can take being an "adult" too far, however. They can say that anything that doesn't help you earn money is a waste of time, that anything creative is also a waste of time, and chase status and money. Maybe many of these people got burned at some point for being childish (e.g. they were criticized by a manager or partner and made to feel shame), which explains the behavior.
It depends on your goals, but a nice moderate view is to discard with the labels, and aim to be responsible and have self-control, while also being curious and open to creative pursuits. It's also great to enjoy whatever media you like without caring about what others think (whether it's obscure or mainstream).
As with everything else, try to reach eudemonia. Taking yourself too seriously is a personal failing, but having zero responsibility is equally incorrect. Finding a good balance between being an adult and staying in touch with your childhood is essential.
I know a few "NEETs" in my life that are "manchildren" and few of them are happy, and I know a few workaholics that also feel miserable. The thing is, success does indeed make people happy, and so does free time. To feel satisfied one must find work that feels somewhat meaningful, or at least shows some kind of success, and have enough time to enjoy life itself.
TL;DR: too much of anything is a bad thing. Being a manchild or a workaholic is equally bad. Find a balance between fun and work.
> So they shaped a false and convenient image of what it means to be an "adult", such as putting work above family and personal life.
I think there are also substantial geopolitical forces in play. Consider the hippie movement in the 60s as an example: yes it's true that there was no shortage of silliness and plain dumb ideas, but there were also a lot of genuinely enlightened ideas. But letting those bloom and spread at that point in time (considering what was going on geopolitically) could have had genuinely disastrous results. But then at the same time, if it hadn't been snuffed out the very opposite could have also occurred, in which case we might now be living in a much better world than the one we ended up with.
