> Why do some “gym rats” build their muscles well past the point of being a way to attract potential mates?
Being a gym rat is not a heritable trait, but the ape who keeps doing whatever they were doing that made them strong and sexually desirable could be much more evolutionarily successful than the ape who stopped and became weak and undesirable. More generally, a positive feedback loop for beneficial behaviors is a desirable trait.
> Why are some people willing to free solo climb icy mountains despite the danger?
Being a free solo climber is not a heritable trait, but exploring your environment and incrementally pushing one's limits are both desirable traits. The ape that doesn't know which rocks he can climb falls to his death, the one who knows where to place their hands survives to reproduce.
> Why are some people willing to have fewer children in order to pursue career goals?
Corporate ladder climbing is not a heritable trait, but investing more resources in fewer offspring is advantageous when the odds of those offspring surviving are high. Even with no kids of your own, you pass on your genes through nieces, nephews, etc. The desire for resources has obvious evolutionary benefit.
> Why are most billionaire tech CEOs interested in space travel and extraterrestrials?
Neither being a tech billionaire nor being interested in space is a heritable trait, but the desire to explore is undoubtedly a desireable trait. The apes that expand into virgin territory can acquire more resources and become more resistant to local catastrophe than the apes who remain in one location. Elon Musk just has much better odds of becoming king of mars than I do. 500 years ago this may have manifested as building big ships instead of big rockets.
Of course random mutations happen, but those random mutations get eliminated quickly if they are harmful and spread quickly if they are helpful. Saying something is just due to random mutation is not only an unsatisfying answer, it is the answer to the wrong question: it doesn't matter why we started doing it, why do we still do it? Evolution doesn't create any trait, it merely perpetuates or fails to perpetuate them.
Not all behaviors are hardcoded into our DNA, certainly not at the level of specific actions we typically take day to day - my ancestors did not pass on their genes more successfully because of their affinity for arguing on the internet - but those traits which are heritable are unavoidably influenced by evolution. Evolution is the null hypothesis - if a system left to its own devices would do something without any external intervention, why invoke any additional explanation? It is those saying that something is genetic but which has somehow defied evolutionary pressure which are asserting a narrative, and who must show evidence in support of that narrative. Some narratives are legitimate, for example the traits of domesticated plants and animals aren't explained by natural selection; but oftentimes it is just "magnets, you can't explain that!"