Please explain why is it risky to administer physostigmine to all patients presenting with muscle weakness?

Bad idea. Physostigmine is a powerful drug with many nasty side-effects. The vast majority of people presenting with muscle weakness do not have undiagnosed myasthenia gravis or anticholinergic poisoning. Give it to somebody heat exhaustion and you'll further mess up the electrolytes. Give it to some who's having a stroke and they may throw up and aspirate. Botulism / neuropathy? It can kill them.

Related Questions

Why is it risky to administer physostigmine to patients presenting with muscle weakness?

Potentiation. Great question! Certain causes of muscle weakness, such as those caused by organophosphates and succinyl choline, are made much worse by physostigmine, and this can lead to a respiratory crisis. If the cause of weakness is not known, this medicine might be avoided. Hope that helps!