They can still cash in on fads. It doesn't matter if they exist in the lore or not, werewolves, vampires, and zombies are the current big fad in pop culture, and Bethesda is cashing in. Saying that these things are in the lore is just an excuse.
Note that I am not saying they don't have a right to do this, cashing in on the werewolf/vampire fad is just one of the reasons Dawnguard was released when it was; one of many. And frankly, it's not a reason I hold against them. If anything, it means Bethesda is doing research on their consumer base and targeting their content themes towards what they believe will make the most money. This money, in turn, is what pays for the next Elder Scrolls game. Or perhaps an actual QA department; one can dream.