Reason one is that it's easy to write a relationship around two characters that the viewers are already invested in and familiar with. Secondly, it allows a greater amount of tension in scenes with just the two characters that builds interest.
That is the real reason.>>97790017
It's possible if you're socially well adjusted.
Half this fucking thread is regurgitating tired ass "truisms" about platonic friendships or projecting. Some people have acquaintances or friends of the opposite sex that they don't have any interest in sexually, some have close friends of the opposite sex that they see in a fraternal or sorroral light. It happens.
The stereotype exists that it's impossible because guys - warning this is going to sound SJW as fuck - are trained by society to see any relationship with the opposite sex where a woman relates her emotions to him as indicating romantic interest, whereas ladies often express their emotions to each other. Men are usually told to suck it up and expressing themselves beyond anger or machismo is "gay" or weak. Emotional exposure is platonic to women, but because only women are "allowed" to do it, and most men usually only have male friends, it becomes a false indication of a woman being interested in you as a romantic partner.
Fuck that was probably rambling, but it's sexism against men, fucking us up and telling us to believe that platonic opposite sex friendships are impossible.
All that said, I have some female friends that I have no interest in romantically or sexually, I have way more male friends, but that just goes with the nerd ass /co/mrade niche.