In ancient times, the hierarchy was strict and clear. Waiters were workers at the bottom and did not have formal names.
In a store, the biggest leader that customers see is the shopkeeper, followed by the waiter, so it is natural to call them waiter.
As for why it is a man, it is easy to understand. In ancient times, women were not allowed to show their faces. In addition, waiters had to deal with many customers, and female waitresses had many inconveniences, so only men could come forward.