Because the elders preserve Israel and are sought for their wisdom. Rabbi Akiva said:
Why is Israel compared to a bird? Just as a bird can only fly with its wings, so Israel can only survive with the help of its elders. [Ex. R. 5:12]
Rabbi Yehudah said they are owed respect even after they lose their faculties:
Be careful [to respect] an old man who has forgotten his knowledge through no fault of his own, because both the whole tablets of the law and the fragments of the broken tablets of the Law were placed in the Ark of the Covenant. [Berachot 8b]
There is a touching story in the Zohar [re Shemot 2:183a-187a] about Rabbi Shim'on bar Yochai seeing an old man, saying to his companion: "We shall certainly hear new and instructive expositions [of the Torah] from that old man." and accelerating to overtake him. Here you have one of the greatest scholars of his generation rejoicing greatly at the sight of an old man, confident that he has a lot to teach them, and approaches him with great enthusiasm!