Why is atomism described as a realistic philosophy?

Atomism is described as a realistic philosophy because it posits that atoms exist in actual reality, rather than serving merely as subjective constructs or mental models designed to help humans better understand natural phenomena.1

In this philosophical framework, the material universe is understood to be composed of minute, immutable particles. Because these atoms are considered to have an objective existence independent of the observer, the theory asserts that all observable changes in the world are the direct result of the physical motions and shifting configurations of these real, underlying components. By grounding the complexity of the observable world in the existence of these fundamental, indivisible units, atomism provides a mechanistic explanation of nature that rejects the idea that such particles are simply useful fictions.21