The reason you don't see much porn on tubes being advertised is because it converts much worse than dating, penis pills and pickup type sites and the users are worth less. Its about the value of each click you pay for.
It's about user motivation, nothing else.
Adult sites have essentially became a loss leader for porn in terms of advertising because when given the choice of meeting someone that day to have sex with or to masturbate, it's a no brainer for the user and basic biology and the unconscious drive to reproduce takes over and starts making the decisions. The single largest delusion we live with day to day is that which is created the illusion of free will and the extent to which we falsely believe our decisions and actions are consciously driven.