Bruce,

I only know enough about this to be familiar so you might have to find an expert somewhere, but my understanding is that its a matter of effeciency, and detectability.

DoS attacks, because they work at the Transport layer (TCP and UDP are transport protocols) affect ANY Internet machine. Also, the spoofing of the IP Addresses in the header allows the perpetrator to hide their location. If you were to initiate an SQL session intended to tie up a machine, A)it would be detectable because the packet origins could be known, B)Only affects machines that are running the Server you are attacking, C)Would require a more significant investment of machine resources by you the attacker, etc.

In short, they don't, because they don't have to. The transport protocols are a juicer target.

HTH,
Bill