The problem is the temperature limit on the heating elements. You could use arc heating, but that's not very efficient and also causes erosion of surfaces (and demands high instantaneous power; this scheme stores energy as chemistry).

The temperature of gas in a conventional rocket engine is high enough to melt or vaporize the thrust chamber; only cooling and limits on heat transfer to the walls keeps that from happening. In the SpaceX Raptor engine, for example, the gas reaches a temperature of at least 3500 C. In comparison, the boiling point of iron at 1 bar is 2870 C.