I'm sorry, but that doesn't explain the question I asked. I stated that I understand why these products sell out. That part isn't complicated.
The part I don't understand is why I never see them re-supplied for even a few minutes before selling out again.
Think of it this way. How long do you think it takes for a roll of toilet paper to get to a store from the time an order has been made?
Toilet paper is a a cheap commodity that lasts forever with a usual very consistent demand (in normal times, the amount of toilet paper sold stays constant and predictable from month to month).
The supply chain is set up to minimize the shipping costs and the warehousing costs. You want to pay almost nothing to get it from the factory to the store. And you want to have just enough around to cover the needs of the store so you can use valuable space in your warehouse for something else.
So all of a sudden there is a very odd, completely unexpected, spike in demand. This disrupts what is normally a well managed rather boring supply chain. Orders have to be made, factory schedules need to be shifted.
Then your toilet paper likely gets put on a container ship, where it floats across the ocean for several weeks. Then it gets picked up by a truck and only then it gets to the store.
And then there is the matter of risk. If you are a factory owner who hears that crazy Americans are buying ridiculous amounts of toilet paper. Do you switch over your factory to make toilet paper for them? If you do this you might make money, or the crazy Americans might stop buying toilet paper and then you are stuck with a product you can't sell.
The supply chains for inexpensive products are carefully designed to lower costs. They can't be switched on and off in a day.