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).
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.
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.
I read that growing up means finding yourself in circumstances where there are often no other "adults in the room," but you can step up and take the responsibility to manage any uncertainty.
Ice cream and pizza taste better than salad. If you let them, children will always pick them. Adults know that doing that all the time will actually make them sick and miserable, so they start making choices that are less pleasant in the immediate term because they want to optimize value for the long-term.
Sure, some things are bullshit even in the long-term, but most of the things that seem "boring," like doing the laundry and paying the taxes and making the meal plans, are actually ordered toward optimizing happiness on a much larger scale than children have the life experience to see.
They don't have enough perceived capital to start a family, hence why so many give up and spend it.
And in several countries, saving was a net negative on one's ability to purchase a home unless you lived immensely frugal. For several years, potentially still ongoing.