The amount of energy required to force that amount of gas into the cylinder is immense. Causing a large enough pressure difference to force the piston down and drive the engine, and also the vehicle, would take a huge amount of energy. So they question isn't so much why can't they, but where would that energy come from?
The energy in a standard internal combustion engine comes from the fuel, whether thats ethanol, gasoline, or diesel. The fuel combusts and releases a large amount of energy in the form of heat, which causes the air to heat and expand. The heated air in the same volume of space creates a high pressure, which forces the piston down.
What's advantageous about internal combustion engines is that the fuel they use has a very high energy density. That is, they have a lot of energy for the amount of space and weight they take to store. Electric motors are great in the sense that they are incredibly efficient. While the batteries are currently are no where close to the energy density of more traditional fuels, there is much less energy lost in their use to heat and friction.
In your proposed system, the gas injector would be doing all the work of creating the pressure. It would need to be seriously strong, and have a large amount of energy behind it to do all this work. It's not impossible, but it would likely be much less efficient than anything that currently exists.