Here's why:

* Divergent boundaries are where tectonic plates move apart.

* As the plates pull away, the crust thins and weakens, causing the land to sink and form a valley.

* This process is often accompanied by volcanic activity and earthquakes.

Examples of rift valleys:

* The East African Rift Valley is a prime example, stretching thousands of kilometers across eastern Africa.

* The Mid-Atlantic Ridge is a massive underwater rift valley where the North American and Eurasian plates are pulling apart.

Let me know if you'd like more detail on how rift valleys form!