Why don't ion thusters Directly use exothermic reactions to heat up plasma and Ionize gas?

Ion thrusters require a lot of energy, because of their high exhaust velocities. But why can they just pump a mixture off hydrogen and xenon into the chamber and mix it oxygen and burn it up. Doesn't this ionize the gas and also provide enough thermal heat to push the plasma through the grids?

• Maybe not hydrogen and oxygen but any exothermic reactions could be utilized. Not to completely avoid electricity input but partially. Reducing the size of the solar panels maybe. I am talking about deep space missions.

-This would also be much more efficient than energy production through turbines, wouldn't it?

-Then why aren't they doing it? Is there any drawback of using exothermic reactions?

• See this answer to the same question by the same user posted only 40 minutes later. – uhoh Feb 8 '17 at 6:05

By using a separate energy source, the ion thruster can get the propellant moving faster. Ion thrusters can get to around 29,000 m/s. This directly relates to how much velocity change ($\Delta v$) you can perform from a given mass of propellant.
I think your question is why not do both? Why not have combustion aid the process? The reason is that the mass of whatever you're combusting reduces the propellant you can take. In missions where ion thrusters are selected, this would not usually be a good choice. If the engine itself could take it, it could increase the thrust, but only at a cost of lower $I_{sp}$ and lower fuel mass efficiencies.