Personally, it's because I know other people. Having gone through artificial panics and unreasonable demand-based shortages based entirely on fear I know that my fellow human being is often irrational, selfish and short-sighted when left to their own devices. The _only_ solution to resolving those periods of irrationality I experienced were limits placed on their behavior like rationing of gasoline or toilet-paper or water or etc.
Another solution is to not worry about it. Rational people will have enough toilet paper to survive the shortages caused by panic buying of irrational people. You did have enough toilet paper, didn't you? Or were you short-sightedly assuming the shops would always have what you wanted when you wanted it?
