Here's why:

* Composition: Concrete is made by mixing cement, aggregates (sand, gravel, or crushed stone), and water.

* Formation: Rocks form over geological timescales through natural processes like sedimentation, volcanism, or metamorphism. Concrete is manufactured by humans.

However, concrete can become rock-like over time:

* Curing: After it's poured, concrete undergoes a chemical reaction called hydration, where the cement reacts with water to harden. This process creates a solid, durable material.

* Time: Over very long periods, concrete can be affected by the same processes that transform natural materials into rocks, such as weathering and erosion.

So, while concrete isn't a rock in the traditional sense, it does share many of the properties of rock and can even become rock-like under certain conditions.