Because it will sell is why. The Ford Edge theoretically makes ZERO sense between the Escape and the Explorer, but it is a big seller. Yes the LWB Tiguan should be "enough" but it would not surprise me if the SWB Atlas sells as well as the Atlas. Sales of the Atlas are pretty dismal for its segment. Even if it doubled its sales it would still be a slow seller in its segment. Thus any market share is good market share if you're the clueless VWAG.
I'd do a new Thing by rebodying a Tiguan as well.