Habit is overcome by habit

Desiderius Erasmus knew that habit is overcome by habit. He lived in the 16th century:

A nail is driven out by another nail. Habit is overcome by habit.

About the same time, Thomas à Kempis was writing that getting rid of a unhealthy habit was not enough, we must replace it with a healthy habit:

We are working each day to completely remake our lives, bit by bit. We are replacing unhealthy habits with healthy ones.

The important knowledge that habit is overcome by habit has been around for 500 years and probably more. It is repeated today as well:

The way to stop a habit is by not stopping it cold turkey but by substituting a new habit. The new habit is a beneficial one and good for you. It could be good for you physically, mentally, or emotionally. It is the course of action, in an ideal world, you would like to take but are are struggling to do it.

A new habit can take up to 30 days of doing it to become established. The first six days of the new habit is hard to follow, the six days after that a little easier, going six days forward again it becomes easier still. As the habit is performed every day it does get easier and better.

If so much is known about the mechanics of forming and removing habits how come we still have bad habits? How come removing a bad habit is not as simple as learning to drive a car? And as free of prejudice?