Here's why:

* Mid-ocean ridges are where new oceanic crust is created. Molten rock from the Earth's mantle rises up, cools, and solidifies, forming new crust.

* The crust gets older and thicker as it moves away from the ridge. As it moves, it cools and becomes denser, sinking slightly. This process is called isostatic adjustment.

* The thinnest crust is found at the ridge itself. This is where the new crust is still very hot and relatively thin.

Let me know if you have more questions!