a) because it's a different business model. not all companies have the resouces to build their own sites from scratch.
b) becuase the adult market is already over saturated and it is perhaps better to service existing companies than try to create your own from scratch.
That's like saying "well why do all these companies sell their product to walmart? why don't they go out and just sell direct? they can make more money that way"
Just because you're good at something doesn't necessarily make it feasible for that company to expand into other markets.
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