if you look at the title it says gaming machinesso in this case your not using the onboard gpuyoued :
why use the 2600k if it cant oc?
Because some people WILL use onboard video? The 2600K has twice the GPU power as the non-K. Besides, you're not asking the right question.
The right question is, why do no LGA 1155 ITX boards overclock? And the answer is "because Z68 isn't out yet".