Why is silicon used in computer chips?
Most electron components of computer chips and boards are semiconductor devices like transistors and diodes.
Silicon is, from the Wikipedia article on semiconductor devices:


  By far, silicon (Si) is the most widely used material in semiconductor devices. Its combination of low raw material cost, relatively simple processing, and a useful temperature range make it currently the best compromise among the various competing materials


Silicon is the second most common element in the earth's crust after oxygen, and present in a variety of silicate minerals.

Electronics grade silicon can be produced easily by molten salt electrolysis of these minerals followed by purification by zone refining.

Copper is a conductor, and therefor not suitable for semiconductor devices. Tellurium is one of the rarest elements in the earth's crust.