Everybody in a busy restaurant works hard, but where tipping is traditional the work is not rewarded equally.
On the tables, one gets paid better for working harder. In the kitchen, this doesn't happen.
If an environment is progressive enough, the cook will get a small percentage of a pool of the tips that the wait-staff is made to share but it's a pittance.
And, since the "tip-out" is a percentage of the gratuities that the waitstaff get and report, the amount received in the back of the house is not based upon performance there.