Here's why:

* Compression: When tectonic plates collide or move towards each other, they apply immense pressure on the Earth's crust. This pressure causes the rocks to bend and fold, rather than break.

* Ductile Deformation: At significant depths, the rocks are under high pressure and temperatures. These conditions make them more ductile, meaning they can deform without breaking. Folding is a result of this ductile deformation.

* Types of Folds: There are different types of folds, including anticlines (upward folds) and synclines (downward folds), which form depending on the direction and magnitude of the compressive forces.

So, folding is a common feature in areas of active mountain building, where tectonic plates are colliding, or where the crust is being compressed.