There are two parts to it. The seller notifies the state that they have sold the car, and the buyer has to register with the state that they bought the car. Taxes are due when the buyer registers with the state, so guess which part doesn't get done as often?
If the seller forgets to send in the sell notification and the buyer flakes out on registering the only person they know to pursue is the seller. Hopefully the seller has a signed bill of sale so they can point the finger further down the ownership chain.