... why do we call the cables connecting the gas and water back to MET - the main bonding conductors?
Because these services are earthy and would remain so in the event of a fault in the electrical installation and thus represent a hazard.
So, although we don't have to earth them, we bond them to the earth reference point of the installation, namely the MET.
They are the main protective bonding conductors because they establish an approximate equipotential datum.
Supplementary bonding may be needed to, er, supplement or re-establish this 'equipotentiality'* elsewhere in the installation.
(*See also equipotentialness, equipotentialitude & equipotentialersity.)