There is definitely such a thing as a friend zone.
It is possible for there to be a parallel where you might find a friend of yours very, very attractive and have fantasies about nailing them, but the fact you've been friends for so long can mean it would just be too, for the want of a better word, 'weird'.
In fact, to take it further, you could even have a situation where not only do you want to get into their pants, but you also reckon they wouldn't be half bad in a relationship sense, either.
But again, in parallel with that, you just know you've been 'just friends' for so long that it's very hard to conceptualise of it happening in practice.
Of course there are always exceptions to the rule, but I reckon that's the way it often works.