Why does fighting drain so much stamina/energy.

[Larry the boss]

Anaerobic respiration (Boxing) vs Aerobic respiration (jogging).

Boxing demands a high level of energy within short intervals. Your body will go through anaerobic respiration as a result; it will burn through energy faster to meet the demand. The reverse is true for endurance activities that require a steady and moderate amount of energy.

If you were to sprint at your top speed for 100m, stop, and then do the same thing again until exhaustion, you'd find that the distance travelled would be less that the distance you could travel by simply jogging.

Hence why interval training burns more fat and builds more stamina

[TRTboy]

Because it's a very stressful situation.