Because men are admittedly an easy market. We're simple creatures of simple needs (who actively choose the least simple route of approach 50% of the time). One of those baseline desires is the obvious: sex/sexual gratification. Easy way to get it? Unfortunately, porn. This is part of the reason porn addiction is on the rise: sex has become an easily marketable commodity, whether or not it's actually physical being irrelevant. Where there is demand, there will inevitably be an opportunity.