> Why do some “gym rats” build their muscles well past the point of being a way to attract potential mates?
Margin of safety. Many would probably disagree with the author's assessment that they're well past that point.
> Why are some people willing to free solo climb icy mountains despite the danger?
Conquering new land is deeply embedded into our psyches for evolutionary reasons. The species that's not afraid to do that will probably be more successful in the long run. Conquest of more land is generally correlated with better survival chances.
> Why are some people willing to have fewer children in order to pursue career goals?
Perhaps many, like myself, are aware of how expensive kids are but unaware of how difficult having kids gets later in life. So they optimize this wrong. I wish I had started earlier, but I put it off until later because I thought I'd be able to provide for them better. This thinking was correct, but having them earlier would still have been better, for me at least. The point is, it's certainly not due to a lack of evolutionary interest in having kids.
> Why are most billionaire tech CEOs interested in space travel and extraterrestrials?
See conquest of land above...