Why is silicon used to make microchips?
Any of a number of semiconductor materials can be and are used, indeed the first transistor was actually a Germanium (Ge) transistor.  the real reason why Si is so dominant comes down to 4 principal reasons ( but #1 is the primary reason):

1) It forms an oxide that is of very high high quality, seals the surface with very few pin holes or gaps.
- this allows gap MOSFET to be more easily made as the SiO2 forms the insulating layer for the Gate,
- SiO2 has been called the chip designers friend.

2) It forms a very tough Nitride, Si3N4 Silicon Nitride forms a very high bandgap insulator which is impermeable.
- this is used to passivate (seal) the die.
- this also used to make hard masks and in other process steps

3) Si has a very nice bandgap of ~ 1.12 eV, not too high so that room temperature can't ionize it, and not so low that it has to high leakage current.

4) it forms a very nice gate material.  Most modern FET's used in VLSI (up until the latest generations) have been called MOSFET but in actual fact have used Si as the gate material.  It turns out that it is very easy to deposited non-crystalline Si on surfaces and it is easily etched to great precision.

Basically the success of Si is the success of MOSFET, which with scaling and extreme integration has driven the industry.  Mosfet's are not so easily manufactured in other material systems, and you can't drive the same level of integration in other semicondcutors.

GeO2 - is partially soluble

GaAs - does not form a oxide

CO2 - is a gas

Semiconductors are used because with selective contamination (called dopants) you can control the properties of the material and tailor it's operation and operational mechanisms.