Its because people stick a set of AVOs on their old fiesta, and wonder why the ride is rubbish.
When you deal with real suspension, and adjust it properly, no reason for it to be boneshaking.
For most little boy racers who indulge in el cheapo suspension, a decent setup would end up costing more than their cars.
Case in point:-
http://www.nitron.co.uk/catalogue_item.php?catID=12148&prodID=73643
Its like people "upgrading" their brakes using people like "HiSpec" and wondering why AP Racing, Performance Friction, and Brembo are more expensive.