Why Are Optical Telescopes Placed as High as Possible?

The reason large telescopes are built on tall mountains or put in space is to get away from the distortion of starlight due to the atmosphere. The atmospheric distortion is poor seeing, reddening, extinction and the adding of absorption lines to stellar spectra.

Why are large telescopes often placed on top of mountains?

Optical astronomers put their telescopes on top of mountains to get above the cloud level (as far as possible) and to get away from the light pollution of cities so as to get the best view of the night sky. Also if it is atop a mountain then there isn't some big thing like a building or trees etc blocking the view.

Why are optical telescopes located in uninhabited places on mountains?

Optical telescopes are located in uninhabited places on mountains in order to avoid the distortion of starlight caused due to the atmosphere. This distortion interrupts the observation causing poor sight, reddening and addition of several absorption lines to the spectra.