Why do car glass gets mist while driving during rain?

Assuming you mean mist on the inside surface of the glass:

Typically, when it is rainy, relative humidity is very high. Your own respiration produces water and increases the level of humidity in the air inside the vehicle cabin.

The glass is exposed to water. As the car moves, air flows over the water on the outside surface of the car causing some of this water to draw heat from the glass and evaporate, lowering the temperature of the glass.

Being cooler than the interior air, the glass absorbs some heat from the interior air cooling it slightly near the interior surface of the glass. The high humidity and lowered temperature of the interior air adjacent to the glass causes water to come out of solution (it is dissolved in the air) and deposit on the interior surface of the glass.

To alleviate this problem:

If you have Air Conditioning: Turn on the defroster or air conditioning with the temperature set to anything other than either full hot or full cold. A mix (any mix) is better. Choose a mix that gives you a comfortable temperature. Set the system to Recirculate (not Fresh Air) if possible and keep the Windows closed. The air inside the car will pass over the cold AC Condenser and water will come out on solution there, becoming cold and dry. This air will then pass over the heater core and become a bit warmer and remain dry. This warm dry air will not deposit water on your glass.

If you do not have air conditioning:

This doesn't work as well as with air conditioning, but here goes: Turn on the defroster at high fan speed and temperature as hot as you can use without becoming uncomfortable. Open the rear Windows (not the front ones) slightly, or if not equipped with openable rear Windows, then use the front Windows. Use two Windows. Warm air from the defroster will heat the glass and the slightly open Windows will help keep interior humidity almost equal with exterior humidity.

Misting on exterior surfaces:

-You may have an oily coating on your glass. During dry weather use dry disposable paper towels without any cleaning chemicals to press and rub hard against the glass. Rub a lot. This will remove oils.

-You may have glass that has been treated with water repelling chemicals that are supposed to make windshield wipers unnecessary. These chemicals can cause this mist. In the future, avoid applying such chemicals to your auto glass if misting is a problem

Misting on interior surfaces:

-The inside of the glass may be dirty, just not dirty enough for you to see it. During humid weather, particled on the inside surface of the glass can provide nice places for condensation to form. To fix, first clean the glass with glass cleaner and paper towels. Then allow to dry completely. Then, rub vigorously with clean dry paper towels.