Why did Ptolemy believe the planets were attached to nested spheres?

Ptolemy believed that the heavenly bodies were attached to nested spheres because he sought to create a physically coherent model of the cosmos that accounted for the circular motions observed in the heavens.12

In his work Planetary Hypotheses, Ptolemy proposed that the eccentric and epicyclic motions described in his Almagest were not merely mathematical abstractions but represented the physical reality of the universe. He envisioned these circular paths as the "equator" circles of three-dimensional, solid, revolving spheres.231

By nesting these spheres inside one another, Ptolemy aimed to construct a complete image of the cosmos where there was no empty or wasted space between the Earth and the celestial sphere. This arrangement allowed him to organize the mechanisms for the Sun, Moon, and planets in a continuous sequence, ensuring that their calculated distances were consistent with his observations. This concept of a gapless, nested universe became a standard feature of medieval cosmology for centuries.123