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I'm no power-plant engineer, but vaguely remembering back to thermodynamics 101, in a perfect heat-engine efficiency is relative to the temperature delta.

You can't just turn heat to electricity, so below a certain threshold it's only good for warming spaces.

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They exist but you're right that you can't just use the waste heat directly. You have to pump in some extra heat to make it worthwhile.

They are called co-generation power plants. The one I used to work at took the waste heat from gas turbines, added more heat with HRSGs (heat recovery steam generators) and fed it into a steam turbine. It adds a significant amount of efficiency.

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