Why do you call your ancestors ancestors?

Answer 1

"Ancestor" is the English word for those from whom you are descended. You use that word because that is what the word is. In many ways words are arbitrary sounds that are agreed to have a particular meaning.

The English word ancestor comes from the Old French word ancestre. This in turn comes from the Latin word antecessor. They all derive from from the Latin verb antecedere, meaning to precede or go before, which is what an ancestor did.

So your ancestor's ancestor is also your ancestor. We do not need or have a separate word for that in English.