Done to prevent all sorts of worries/scenarios expressed in the last 3 pages. The "pressure differences" would stress the aircraft. In addition it would be expensive to build the aircraft with both cabin and hold sealed from each other to maintain such "pressure differences". This additional expense would result in extra weight and extra operational costs and shorter range and...

Practical solution is to seal the aircraft from the lower atmospheric when at some altitude and keep all internal areas at the same pressure. ^