They wouldn’t, and they don’t. The “cleared to land” announcement made by flight attendants is almost always bogus. It usually comes just after a series of signal chimes from the cockpit. But there’s nothing specific about these chimes; they’re merely a way of saying, “We’ll be landing soon.” Actual landing clearance typically comes much later, sometimes only seconds prior to touchdown, and it is not something communicated between pilots and cabin crew, ever. I’m not sure when or how this habit got started, but it never fails to irk me.

Another one you’ll hear is “Ladies and gentlemen we’ve begun our final approach…” To a pilot, approach is a technical term referring to the execution of a published arrival pattern, and final approach is even more specific. Flight attendants use this expression generically for convenience.

And for the record, it is not true that a runway must be vacant for a flight to receive landing clearance. Airplanes are cleared to land all the time when arriving or departing flights are still on the strip. It simply means they may go ahead and land without further communications with the control tower. If the runway is not vacant in time, the clearance will be canceled and the plane will go-around.