The reason: pure projection by both sexes. Guys want to have sex with their girl friends, and assume girls feel the same way. Girls do not want to have sex with their guy friends, and assume guys feel the same way.This is largely true, but the logic obviously permits one exception, and it is an exception that I have personally observed. Men of higher SMV can be friends with women of lower SMV unless they convert the female friend into a harem member.
I've had a few genuine female friends with whom I've never had any romantic involvement, three of whom were even attractive. But in all three cases, my interest in them was either totally nonexistent or very limited. In the one instance of the latter case, friendship was possible because her interest in me was equally limited, my being at least 100 pounds too light for her. She was so predictable in this regard that if she was cheerful about a new prospect, I would quite literally ask her at which major football program he had played. She would get mad, then, when pressed, reluctantly admit "Nebraska" or "USC". She very much liked those big, corn-fed linemen.
The reality is that most men aren't truly friends with women, nor can they hope to maintain their friendships once their friend pairs off with another man. Unless the man's SMV is much higher than the male friend's SMV, he simply can't afford to tolerate the friend lurking about and waiting for his opportunity to make a move.
Regardless, the reality is that even when male-female friendships are possible, they tend to be transient and situation-based. I don't know a single man or woman who has maintained a lifelong friendship with a member of the opposite sex that is even remotely comparable to their lifelong same-sex friendships.