Here's why:

* Plate Tectonics: Continental drift is driven by the movement of tectonic plates, which are massive slabs of Earth's crust and upper mantle. These plates move on a layer of molten rock called the asthenosphere.

* Speed of Movement: The plates move at an average rate of a few centimeters per year. This is roughly the same speed as your fingernails grow.

* Millions of Years: It takes millions of years for continents to shift significantly. For example, the Atlantic Ocean, which is widening as the Americas drift away from Europe and Africa, is opening at a rate of about 2.5 cm per year. This means that it takes millions of years for the ocean to grow significantly wider.

In summary: While continents are constantly moving, the process of continental drift is incredibly slow and happens over vast stretches of geological time.