Most Helpful Guys
Because they think that being close with the opposite sex will eventually lead to sexual tension and desire. They don't understand that not every guy is gonna want to bang his female friend. And vise versa.
I have four women with whom I'm friends with. For two of them, there is no sexual attraction, and it's easy to be just friends. (Yes, there is an occasional sexual thought, but nothing that gets in the way) For the other two, there is some sexual attraction at times, but I value their friendship, and refuse to act on the attraction, because the relationship with my girl come first, and because their friendship means too much to me, to ruin it by letting myself give in to an attraction. Also, it works because we have unspoken set boundaries, that we don't cross. We have similar moral beliefs, that make it easier.
Most Helpful Girls
Because to many guys it's hard not to have sex in mind while girls need a lot more to see him as romantic partner.
I had guy friends that ended up confessing their feelings. I was far away from any romantic feelings to them. It ended up weird and now we are not friends anymore.