Why does Knob and Tube need an air gap?
The air gap isn't primarily about heat. It's about electrical insulation. The fabric mesh that was used as insulation was not reliable, and could allow arcing and short circuits (especially if moisture comes into play). It's sort of the same principle you see in overhead power lines.

One could argue that there is a safety factor in the air gap with respect to heat, but the simple fact is that an electrical overload will heat the wires much faster than air could cool them anyway. This could allow the wires to burn or melt, or ignite combustible materials which they contact.