Because in GT6 Heat is just Heat, not limited by the temperature. You can use pretty much any heatant through a Heat Exchanger to produce arbitrarily high temperature on a Crucible, which does not make any real life sense at all either.
And Electric Heaters have no advantage besides easy switching, and their efficiency is low. While in real life they’re capable of reaching higher temperature than what can be reached through combustion alone, and the efficiency loss happens mostly at Heat->Electricity instead of the other way around.
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