I don't get why people keep repeating that being an island is a big deal in a modern pandemic. Infected people came into the US by plane, and general incompete

I don't get why people keep repeating that being an island is a big deal in a modern pandemic. Infected people came into the US by plane, and general incompetence caused it to explode from within. It wasn't caused by some rush of infected Mexicans slipping through the southern border.