[susan53]

First of all, the 3rd person s and plural s are unrelated. They are two different morphemes which by pure chance share the same representation in spelling - just as eg the animal "bear" (noun) is not connected to "bear" (verb) meaning carry. Just because things are spelt the same doesn't necessarily mean they are the same.

The "s" morpheme which is used with nouns indicates plural - ie more than one. It has different variants, including -es (dishes) -en (oxen) and zero (sheep). The -s plural developed from the Old English plural -as

The "s" morpheme which is used with verbs indicates 3rd person singular present tense. It too has different variants, including - es (does) and zero (can). But this doesn't mean it is the same morpheme as the plural s morpheme. It developed from the Old English 3rd person marker -aþ, which by Middle English had become -eth (hath, giveth, taketh). and then changed to -s.

So basically your question doesn't make sense. It's like asking "A person has legs and can walk. A table has legs too - so why can't it walk?" The legs of a person/animal and the legs of a table are not the same thing.