When you're young, you're often engaging in a common animal behavior known as "play". This is essentially practice-mode for life, where you physically or mentally act out a lot of the abstract ideas you've been learning about over the years. This is critical, because our abstract ways of understanding and communicating advanced concepts are still fundamentally incomplete. You can, for instance, teach a kid to be honest, that honesty is important, etc. But then they get into a school environment, surrounded by real life situations. Will honesty always benefit them, like a "good" thing is supposed to? No.

Our abstract understanding of honesty and its importance is one thing. Putting it into effective practice is another, and fundamentally circumstantial.

"Play" is how animals bridge these two things with personal experience, while hopefully avoiding the consequences of actually trying for real and potentially having an accident. Like, an animal could abstractly learn about hunting by observing its mother. But until it actually physically practices these skills, it will be very bad at them. Us learning about "the importance of honesty" is no different.

Humans have a vastly, exponentially greater number of abstracts we're required to understand in order to be effective citizens of the modern world. We tackle them in the same way, though, with play. Play, is practice.

So, if play is practice in an attempt to bridge some kind of abstract, incomplete learning, then what do you have to gain at this current phase of your life, from this "play"?

Your subconscious gets this. You don't need to play anymore, you're good enough for the real thing. So, why should your brain want to play at something? Especially when getting older also makes it clearer just how much incorrect information is being taught in gaming. Like, how many people try to use their CoD experiences to understand the Russo-Ukrainian War?

Anyways, it's complicated.

edit: Thinking further on this, I would propose the following: In the same way that horniness is the mechanism by which your genes make you reproduce them, and hunger is the mechanism it employs to make you fuel their work, "fun" is the mechanism by which your genes make you practice whatever skills or experiences might improve your chances of passing them on, in an environment where it is safer to do so.

This is why play gets fundamentally less fun as you get older. It begins to lose its purpose, outside of handing those skills, and the techniques for practicing them, on to the next generation. We prefer to go back to those same games we played though, because we're refining the lessons we learned from them. This has an evolutionary benefit as well, actually, as even our methods of "play" can be improved through long enough practice and iteration. These refined methods of play can then be handed down instead, which will likely be more efficient than previous iterations.