Because porn sites are known to be serious, trustworthy entities that definitely don't have a long history of distributing viruses and scammy penis enlargement products, and generally trustworthy ad networks that aren't spooked out by porn as their source of revenue. They totally wouldn't sell user data as extra income.

There totally won't be fake porn sites that needs your credit card and now your ID to verify your age and identity before they show you all those hot chicks in your area.