Ego was a celestial born outside of the influence of another celestial (i.e. he wasn't born of a celestial seed like the ones we now know are) and thus, lacked the typical physical form of one. He didn't have a planet, or sentient energy to fee off of to grow and mature like a celestial born of a seed would.

Is Ego a Celestial being?

Celestial Physiology: Ego is a Celestial, a primordial race of entities with vast cosmic power, capable of manipulating matter and energy. Ego lacks a true body, his "core" being a glowing blue brain that houses his consciousness.

How is Ego a Celestial marvel?

Celestials like Ego are categorized as a highly-intelligent ancient race of entities with god-like power. Ego figured out how to manipulate matter and energy around him, which he used to create his own planet as well as a humanoid form that would explore the cosmos.