Wouldn't the net result of what you're proposing be a large number of aircraft all over the country lined up on a short final without an actual landing clearance?
In addition to making things stressful for the flight crew, wouldn't it also put a large amount of stress on the controller, who now needs to issue landing clearances in a short / timely manner regardless of whatever else he is doing or who is is talking to, as soon as the ground aircraft are clear.
I would think it's just easier to do on the initial call - short, sweet, efficient, aircraft calls and he's taken care of one transmission later.
Works fine 99.9% of the time, for the other .1% they can issue a go-around.
..which is also another interesting point, as presumably the aircraft would have been on short final anyways, even if the controller used the method you described. Controller wouldn't have issued a landing clearance - the net result would be the same, issued a go-around, aircraft didn't hear/listen, landed anyways. Would certainly be a little more paperwork for the pilot though.
