Old and Righteous - why commanded to respect?

 Shemos Rabbah (31:16) THOU SHALT NOT REVILE ELOHIM. R. Meir said: Above all did G-d exhort Israel concerning their judges who teach them justice and concerning their ruler, for it says, THOU SHALT NOT REVILE ELOHIM, NOR CURSE A RULER OF THY PEOPLE. You will thus find that Korah and his congregation were smitten only because they stretched out their hands against Moses and Aaron. The citizens of Jerusalem were also smitten because they despised the prophets, for it says, But they mocked the messengers of God, and it is written, They have made their faces harder than a rock; they have refused to return. It was for this reason that God cautioned Israel concerning the respect due to the old and the righteous, as it says, To understand a Proverb, and a figure; the words of the wise, and their dark sayings Why did He exhort us concerning them?--Because it is they who warn Israel against idolatry.