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.

I just realized that heating and expelling something electrically vs just burning in a rocket engine makes absolutely no difference from an ISP perspective. All that matters is exhaust velocity, and that basically only scales with temperature and molecule size (lighter being better), so steam is no good anyway.

Hypothetically though, if you would just pressurize e.g. H2, then cool it, then pressurize it further-- should that not get you higher exhaust velocity that just heating/burning? why does that scheme not work?

What matters is the state of the gas as it starts traveling down though the nozzle. All that comes before that doesn't matter.