It is probably cost. Motherboards are high-volume products, and therefore the engineers who design them are fanatically cost sensitive. Adding $5 to the bill of materials for a feature that hardly anybody needs would be a career limiting move. Conversely, the people who actually need a 10 Gbps links are exactly the people who can be up-sold to a $50 cable.

In a few years it will be everywhere natively: competition will have driven the parts cost to pennies per board, and average customers will be demanding it.