It is the pervading thought that the one being penetrated is of the weaker inclination. Look how women are treated. Nobody wants to be seen as weak, womanly. Gay men are still essentially men and men are hardwired for centuries to be assertive than receptive. And generally, gay men are more attracted to those who are more "manly" since bottoming is perceived as being "too womanly". That mindset is wrong in so many levels. Dominance/manliness is never measured by who does the penetrating or who gets penetrated. And yes, it happens. Bottom jokes are very common and those who bottom are always subject to ridicule, hence this stigma prompts gay men to mostly present themselves as top so as to prevent possible discrimination among peers.