That’s what a visa is. Without a visa you can’t even get on the plane (unless your passport is from a country that doesn’t require one). But they also want to

That’s what a visa is. Without a visa you can’t even get on the plane (unless your passport is from a country that doesn’t require one). But they also want to double check when the border crossing happens. A visa can be issued months or years in advance. This check needs to be done by representatives of the government of the destination, so it’s not practical to do it at the originating airport in most cases.