There are a number of different possible answers to that question. One answer is that it doesn't, it just feels like it does. And if you look at studies of people who are caffeine dependent, they're not actually are more alert after drinking a cup of coffee than people who aren't caffeine dependent and don't drink coffee.

The other answer is that if you're using caffeine from plants then you're getting a different dose each day, and if you're not taking a lot and you're not doing it every day then tolerance will be less of an issue. Also, caffeine breaks down into multiple separate drugs when digested so that may also have some impact on tolerance.