I think it mainly has to do with logistics. Some of the races are really close together (like US and Mexico) so the teams start packing the cars into crates immediately when the race ends so they can get to the next race on time. If every team had to have the plank check be done on their cars, some of the teams would have to wait a long time until the FIA officials finally get to their car, remove the plank from the bottom, and use take the measurements to check the legality.