Because we have always done it this way

If you take 5 monkeys and put them in a cage with a bunch of bananas hanging from the celling with a ladder leading up to it. As a monkey gets hungry and wants a banana he will climb the ladder, When this happens you spray all the monkeys with really cold water. After a while no monkeys will go up the ladder.

When this happens you replace one of the monkeys. When the new monkey tries to go up the ladder the other monkeys will attack him and stop him.

When this happens and he no longer tries to go up the ladder you replace anther monkey. You repeat this process untill all the monkeys have been replaced.

If you ask the monkeys why no is allowed up the ladder to get a banana they will say, "We have always done it that way"