More to the point, are people really paying $51,000 for 3 year old cars?
These are very capable SUVs that are no longer in production (either Touareg or TDI). They can easily tow 7700 lbs. without issue, whereas the next capable SUV can manage around 5000 iirc. They are well equipped, have a great engine, and get quite decent mileage. There are very few similar SUVs in comparison.
Sure, you could find a cheaper SUV, but as previously pointed out, the Touareg comes with an incredible engine warranty, tows more, features, etc.
If you don't care about tow load, fuel efficiency, or warranty; perhaps you're better suited to purchase a used Exploder, Santa Fe, Yukon, etc.
A car is worth whatever someone is willing to pay for it (see Barrett Jackson). If a buyer is willing to pay 55k for one, then it's worth 55k. If a seller is willing to accept 30k for one, then it's worth 30k.