Why do people those that those who are panic buying are very selfish people?

They make the prices go up.

People working in the grocery stores have to stock everything back constantly, or replace the empty spot with something else, which means they have a lot more work. 

Grocery stores kind of have a system of how much of everything they order and get delivered. So they get, let's say, x amount of pasta each week, which is a good amount. If everyone starts panick buying on the same day, they basically take away the pasta someone else was meant to buy. If nurses can only shop at let's say 7pm after a long shift and they see empty shelves... It is kind of selfish.