I was trying to see if there could be an explanation based on an impression of solitary activity, or based on jealousy. However, people who run as a hobby don't get widely mocked for it, despite their hobby being a solitary activity doing an extreme version something many people can do anyway.

I suspect it is a streak of anti-intellectualism; probably historically based on jealousy and perceived privilege, but now just ingrained as a societal trope.