Explain to them that's the rule of grammar. The student asked that question because they learn about the plural S associating it with nouns. This is a teaching moment for you, so now you introduce singular and plural verbs. Explain that singular verbs have the S attacked to it while plural verbs do not.
It may be easier to explain singular or plural sentences, rather keep nouns and verbs apart. If a sentence is about plural subjects the noun has the S and the verb does not. If the sentence is about a single subject, the noun has no S but the verb does. In other words, there can only be one S in the sentence.