Also, there are a lot of things which might not be considered "essential" but quite important. Working in home office, I might need cables, a headset, a power supply. Also, if you consider that people are restricted to their homes for weeks to come, even entertainment/toys can quickly be considered essential.
So Amazon probably does its best to use its available resources in storage and delivery, the categorization of items as essential might not be as clear-cut as it might sound.
And of course, toilet paper is out of stock on Amazon as at the local stores :p