Why do sentences need nouns?

Sentences do not need nouns. Some sentences have no nouns at all.

A noun is a Word for a person, a place, or a thing.

Sometimes a pronoun takes the place of a noun in a sentence.

A sentence requires only a subject and a verb, and to convey a complete thought.

Examples:

Grandmother is coming soon. (the subject is the noun 'grandmother', the verb is 'is coming', the Word 'soon' is an adverb modifying the verb 'coming')

She is coming soon. (the subject is 'she', a pronoun; there are no nouns in the sentence)

Sometimes a sentence can be just the verb. In many imperative sentences, the subject is implied.

Example: Stop! (the subject is implied, for example, "Driver stop!" or "You stop!")