A normal ILS approach is about 300 ft/mile. Many airports have runways that are a couple of miles long, so the center of the Class B could be roughly as much as 1.5 miles from the touchdown point. At your distance, a nominal approach would be at 750 feet. At a mile closer, a nominal approach would be 450 feet.

If the pilot is a little low on the glideslope, you're a little closer to the airport than you think, and your drone is a little higher than you think, you've got a collision. Now, these are all worst case deviations, and it might make sense for the FAA to draw the lines more closely, orient them with the actual approach paths, and perhaps have a lower altitude restriction (maybe 150 feet or so) close to an airport. As it is though, the 5 mile ring is a pretty reasonable, if conservative choice given the potential consequences of a drone hitting an aircraft on takeoff or landing.