This question is asked repeatedly. High heat BBQ paint doesn't not have any elasticity. It can't stretch out and then compress itself back into place. What occurs to your firebox when heat is applied? The steel will expand when heated and then contract as it cools. This is what causes the paint to "flake" off. Not "burn" off, although usually referred as that. Also the reason that this occurs on the firebox end and not the food chamber. The firebox is twice as hot as the food chamber. Example, your firebox can run upwards of 500 degrees to maintain 225-250 degrees in the food chamber. This isn't even close to the temperatures that most hight heat paint can resist. Most are 800 to 1200 degrees resistant.
Now, over time, as a pit seasons, the firebox will have a tendency to not expand and contract as much as it did when it was new. Thus, continously repainting will be less frequent.