Here's why:

* Height: Mountains are very tall, but not so tall that kilometers are a practical unit.

* Precision: Meters allow for more precise measurements, which is important when comparing the heights of different mountains.

* Consistency: Most elevation measurements are done in meters, making it easier to compare data.

However, you might see mountain heights expressed in kilometers (km) if:

* The mountain is extremely tall (like Mount Everest).

* The context requires a broader overview of the mountain's scale.

Ultimately, the choice of unit depends on the specific context and the desired level of precision.