That's cool! I do all of those things as well, minus the uni classes and open-source. But for someone who has no interest in travel, then learning foreign languages would be a waste of time. I personally consider writing open-source software a waste of my time because it would be taking away from my closed-source side project ventures.
You can totally min/max your entire life and only do things for self-improvement, but then to what end? What do you expect to derive out of the continuing education classes that is going to be significantly valuable to your life/career?
I think it's important to note that many people are actually quite satisfied with their position in life, a lot of us are making a lot of money engaging in interesting work, so at a certain point doing things for the sake of minimizing opportunity cost comes with diminishing returns as you've already made the most of the opportunities you've already had. The whole point of working hard is to reach some stage of life where you can actually reap the rewards and have the time and resources to enjoy yourself however you please without worrying about it.
I don’t really use my language learning for travel—nearly everyone speaks better English than I do their native language.
But in the same way that learning Haskell or Lisp can change the way you write and think about code, learning a language like Japanese can fundamentally change the way you think about everything. There’s intrinsic value to being multilingual.
Of greater pragmatic benefit is that if you learn a language, like Arabic, Chinese, Spanish, or French, you gain exposure to a whole different media landscape, including information on news and world events that is drastically different from what you encounter in the anglosphere.
What makes any of those things more or less valuable than playing a video game?
Volunteering, sure, but the rest are just as self indulgent as any other pass time. Just because you value them higher doesn't mean you should discount the things that other people do to enjoy themselves.
In our intensely competitive capitalist society, gaming has a low expected utility. It also doesn't usually reinforce skills that make you more employable. You also usually don't get a salary or remuneration.
The elitist in me wants to say games are little more than hi-res Skinner boxes. But properly applied, games can teach us strategy, reason, and problem solving. Is that why I play God of War? No. I play it because KRATOS SMASH
All of those activities enrich my life and help me grow as an individual. I think someone should always be learning throughout their life, and I have yet to see a video game that accomplishes that.