Why doesn't the Explorer Cab come with a jump drive, when smaller ships already in the game have that capability? Can a jump drive be added aftermarket?

There is no slot for an aftermarket jump drive. From a game design perspective, this is important because every ship should not be able to do every job (else, why operate a dedicated explorer ship?); in the game world, the intention is that the Explorer cab uses all its space for armor and shielding, and does not have the power plant to also support a jump drive.