The problem is life support systems: heating and pressurization.
Each passenger pod has to have an independent life support system, to keep the passengers and flight attendants alive while they're sealed into their pressurized spam can.
Each carrier airplane has to have its own life support system, to keep the pilots alive while they're taxiing and driving their spam can carrier.
Depending on the spam can sizing and count, those life support systems together will likely add up to MORE weight and volume than would be required for a comparably-sized unitary airplane.
Also note that the structural problems for handling multiple detachable spam cans are quite a bit different from the structural problems of building a large unitary airplane. (You would not believe me if I tried to tell you what military airplane designers go through, figuring out how configurable external stores affect an airplane.)
And there are other safety issues: If you have a problem in one spam can, and you jettison it, what does that do to the flyability of the carrier airplane and its remaining UNBALANCED load of spam cans? Or do you jettison a perfectly good spam can, to avoid the unbalanced loading? Or maybe if you have to jettison one can, you have to drop ALL of them - at which point there's no real advantage to detachable spam cans and a spam can carrier airplane over the original unitary airplane.
For that matter, you also have to ensure that a life support problem in one spam can doesn't affect any of the others. If the main power bus to the can shorts in the can, you have to be able to cut it, so that it doesn't disable the main power bus to the carrier airplane. (More cans, more failure points, more requirements to contain the failure.)