The coefficient of friction CAN be more than one.
Friction is a force relating to molecular structure of two surfaces touching each other.
Molecules from one surface link with molecules from the other. If you press the objects together more tightly, you push the surfaces closer together, joining more molecules. This proportion is the coefficient of friction. Molecules on adjoining surfaces just don't cling to each other strongly enough to have a huge friction coefficient. Solid molecules prefer to cling to each other rather than other solid materials.