Here's why:

* The Law of Conservation of Mass: This fundamental law of science states that matter cannot be created or destroyed, only transformed.

* Rock Transformation: When rocks change form, they undergo processes like weathering, erosion, melting, and re-crystallization. These processes break down, transport, and reassemble the minerals within the rock.

* Mass Remains Constant: While the shape, texture, and mineral composition of the rock might change dramatically, the total mass of the rock remains the same.

Example:

* A granite boulder exposed to weathering breaks down into smaller pieces of sand and gravel.

* The total mass of the granite boulder is the same as the total mass of the sand and gravel, even though the form has changed.

Note: There might be small changes in mass due to processes like evaporation or the addition of trace amounts of other materials. However, the overall mass of the pre-existing rock will remain relatively consistent.