Why are sedimantry rocks found as a veneer covering large areas of the continantal igneous rocks?

Answer 1

Sedimentary rocks form from the breakdown of other rocks, including igneous rocks. This breakdown, called "weathering", can be and often is a very slow process. So, simply put, there are more igneous rocks that have no been broken down yet compared to what has been broken down.

Answer 2

The igneous rock below are much older than the sedimentary rocks above them. That means the igneous rock have had time to erode and form layers of sedimentary rock.