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Anytime mind if I answer with extra physics? No? Ok. Even thick extension cords are made from many strands of thin wires to be flexible. But, even if you have the same amount of copper, strands have higher electrical resistance. That makes heat, just like your space heater, but it can't handle it.

The plastic coating will melt, it might short and spark. Nothing good. The wires in your wall are single, solid, thick wires that never have to move. They can safely carry a lot more power without problems. The breakers in your service panel will trip before those wires fail (i.e. melt).