Well actually your body does, as we begin to enter into a cardio workout state our brain releases drugs making us feel good and encouraging us to exercise more and push harder.

That's not an absolute in any way, though. If it was, you'd have the same issue on any walk and you'd just wind up sprinting through the forest or local park or whatever as fast as you can because that too is a physical challenge

This is a very obvious aspect of 99.99% of hikers to me?

I don't know what it's like where you are, but that's definitely not my experience. Across both my personal friends and family and random people I bump into while out and about hiking myself, there's a broad mix of the kind of people you describe and people who are doing it in a much more relaxed and casual manner. Why do hiking groups push harder? I don't know, I only ever go either by myself or with friends and family. To borrow the term instrumental play, such instrumentalisation is common across many hobbies. If you start playing a videogame then the online lobbies might be sweaty as hell, but that doesn't stop you playing it casually so long as the game gives you plenty of stuff to do and engage with that doesn't require the online lobby. A lot of people will be playing that game much less instrumentally, but they may be much less visible. I would argue that a mountain or similar does, in this analogy, generally have plenty to engage with other than physically testing yourself