The tent keeps the wind out, the fly keeps the rain off, and the sleeping bag keeps you warm. A good down bag is a lot more effective than an electric blanket. My winter bag is a -20F or so Northface, and it's quite toasty at 5-10F. To insulate an entire tent to that level would require several feet of thickness of insulation - it would fill the back of a pickup truck. (If the square footage goes up 10x, the insulation thickness would need to do the same, for the same heat loss, so the volume of insulation would be 100x.)