>Use one for yourself, and you'll see the cutting flame has to greatly heat up the steel before it will cut.

All the acetylene flame(s) do is heat the iron up to ignition temperature. The "cutting flame" is actually a high pressure jet of pure oxygen oxidizing (burning) the iron in the steel. It also blows the resulting slag out of the gap.

To start the burning, it's necessary to head only a small spot, like a corner of the edge of a plate or piece of bar stock, until it's hot enough to burn, then press the cutting lever. Oxidizing iron releases a lot of heat, and the amount of it is what keeps the "cutting process" going. All the acetylene flames will do by themseves is to melt the steel, provided you play them on the surface of the steel long enough to do this.