For the same reason we have limits on the speeds you can extend landing gear, flaps, and the same limits we have on structural strength, we must have limits on WEP.
In real life a B-17 *COULD* survive a direct hit from a flak and limp home on half a wing and a prayer. Realistically? This is the exception to the rule.
In real life a plane COULD run for 30 minutes on higher boost (also FYI the spit in the example didn't use additives, just higher settings), but what about the ones that didn't? They didn't make it home to tell the tale. The exception proves the rule in this case. Engines overheating and catching fire were common in WW2. Heck, some even caught fire when not in WEP mode.