Well, some do, actually. I'm still very good friends with a guy I first met in kindergarten. And that was a LONG time ago! He lived across the street from me. We were best friends from the time we met. Only just friends, though, not romantically involved at all.
Our senior year in h.s. (different schools), he introduced me to a guy he KNEW would be just right for me, and he was - it was "love at first sight" and we ended up dating for four years, for the rest of high school and most of college.
And THAT guy is also still a good friend, even though we ended up going our separate ways and marrying other people.
My very best female friend is someone I met at age 6, too. We have a long, long history together, through many moves and career changes and all that.
So I'd never put down the possibility of a life-long friendship.
At the same time - if someone is having some doubts about making a life-long commitment to someone, those little warning signs going off in your head are begging to have attention paid to them, because SOME-thing is going on, for sure.