So?
Name me a left back who can magically make Keane a better centre half.
I'll wait.
Your entire argument is based on the idea that any cross that comes into the box is automatically a goal, therefore any fullback who fails to stop a cross is immediately at fault for conceding that goal. You don't seem to be asking "why is it that 50% of the time when a cross comes in the guy Keane is supposed to be marking scores?"
If our fullbacks need to stop EVERY cross from coming in, shouldn't you be asking WHY?