If you put a human in an environment with 20% of sea level pressure, and feed them 20% sea level pressure air, they will suffocate.

If you were to somehow feed them 100% sea level pressure air through a perfectly sealed mask, they will be unable to exhale and/or get some kind of fatal side effects (burst lungs or air bubbles making it into the blood stream). So you have to feed the breathing gas at the surrounding pressure.

If, however, you give them 100% oxygen at 20% sea level pressure, they will be able to happily breathe it as if it was regular air with 20% oxygen near sea level, at least until you introduce an ignition source. What matters physiologically is the partial pressure (pressure multiplied by fraction).

(Likewise, if you give someone 100% oxygen at sea level pressure for a short time, they'll be fine. Do the same at more than twice sea level pressure, e.g. while diving, and the oxygen becomes fatally toxic.)

Incidentally, it is possible to provide the breathing gas at somewhat over the ambient pressure. This can allow survival without cabin pressurization at altitudes above the point where even 100% oxygen wouldn’t be sufficient. But the pressures that can be used are well below what you’d need for normal air to be sufficient at a typical airliner cruising altitude.

> Sufficient concentrations of oxygen can cause even steel to burn

Not just thermal lances; oxy-acetylene cutting torches work by burning through steel, and you can buy one for not much money at almost any hardware store.

> There must be a reason they use pure oxygen, as regular compressed air, also breathable, would not have the same intense reactivity.

Pure oxygen at 1/5th standard pressure has the same effect as air at standard pressure, and assuming nitrogen and oxygen compress similarly you can either fit 5 times more in the same canister, or you can have a canister 1/5th the size and weight.

It's used due to the low partial pressure of oxygen at high altitudes