Because it is not possible - and any intelligent person past their teenage years understands that. What is your next point? :)
"I’ll miss the everyday closeness, the banter, the laughs, the rants. I imagine he feels the same at some level (we don’t talk about things like that)."
That statement alone is brimming with sexual tension.
It is possible for a guy to be "just friends" with a woman that he is close to. But only if he finds her very unattractive otherwise.
It is dangerous for a guy to be "friends" with a woman he finds attractive, and he should avoid opportunities for intimacy.
I’m n a one on one situation you’re are correct in situations where multiple people are involved it’s still possible but not as likely likely
Being on the road traveling consistently with a member of the opposite sex will ultimately lead to problems in the vast majority of situations