Here's why:

* Mechanical weathering breaks down rocks physically, changing their size and shape without altering their chemical composition.

* Chemical weathering involves chemical reactions that change the mineral composition of the rock.

In the case of freezing and thawing:

* Water seeps into cracks in rocks.

* When the water freezes, it expands, putting pressure on the surrounding rock.

* This pressure can cause the rock to crack or break apart.

* Repeated freezing and thawing cycles can significantly weaken and break down the rock.

The process doesn't involve any chemical reactions; it's purely a physical force that breaks the rock apart. This makes it a form of mechanical weathering.