Some people are not adventurous. In the slightest.
I used to do hardcore no-gear rock climbing. I've done skydiving, bungee jumping cliff diving, etc. I didn't do any of that stuff _often_ (except rock climbing in college) and definitely wasn't an "adrenaline junkie" who would do any death-defying stunt that crossed my path. (Wing suit? Nah.) Even though it's been decades since I did any "thrill seeking" type activities, if I was offered the opportunity to go to space? You bet your ass I'd jump on the rocket.
Yet I know multiple people that wouldn't take a free trip to space. Some, even if "the mission needs you" probably would still have to be dragged kicking and screaming. (Also note: in the book, he is one of three people sent on the mission, he has a barely-there legit excuse of "I'm not actually _necessary_ to the mission." Even though the mission really would have been better off with him. Obviously he ended up absolutely necessary and vital; but there wouldn't be a compelling story if he wasn't.)
It was asking a lot of him... Like A LOT.
Great book!