I think I know another reason they use reed switches. that makes it trivial to build computers that are auto-on without drawing much current. With a clean reed switch, the switch can just be hooked to an interrupt line on the microprocessor, and it can go into deep sleep mode and wake when the switch state changes. If it were a hall effect switch it would have to be powered constantly to know when the magnet moved so it could wake up, and the battery would only last a few weeks at best. It might be a little better with a coil but still, it would need more sensitive circuitry that would have to be powered.