Camel carts/wagons actually do exist (http://www.nomadderwhere.com/wp-content/uploads/2009/07/dsc_...) and are quite popular in India and Pakistan. In fact, I would say that such is the primary way of using a camel as a load-bearing animal in this region. I think the big difference is, in the Middle East, the caravans had to travel very long distances (for instance, Mecca to Constantinople was a profitable route) over very _diverse_ desert terrain, some of which is probably very unsuitable for carts (mountain passes, loose sand).