Why do CO2 fire extinguishers do not have pressure indicators?

You should study the subject because there are a lot of different kinds of fires, and different kinds of extinguishers for each one.

LOADED STREAM - A solution of water and baking soda with a container of acid balanced inside. When you turn the extinguisher over, the chemical reaction provides pressure to force the water out through a hose. Not good for electrical fires or fires involving small bits of wood or paper that would be blown about by a stream of water.

BAKING SODA - A pressurized canister. When the valve is squeezed, the pressure forces baking soda into the flames where the heat releases CO2 which puts out the fire. This is the type that has a pressure gauge, also the cheapest type, and the most common failure is a leaky gauge.

CO2 - This type has a slight tendency to freeze when the CO2 is released. One chemical company sells an attachment to capture the dry ice for experiments. This is the best choice for general purpose fire fighting.

There are others, such as one that is so good they only use it inside jet engines. But the gas has to completely dissipate before anybody can breathe to get near the engine. You can check videos.google.com for examples of fuel tank fire extinguishers.