Question

Why do we use runs for a singular noun and
run for plural nouns?

Answer 1

Answer:

Because Singular verb takes +s/es.

Answer 2

We use runs for singular nouns and run for plural nouns because a verb's rules of singlar and plural are opposite compared to nouns. This means run =  plural and runs = singular. We do this keeping the rules of Subject - Verb Agreement's rule.

Hope this helped.