Assembly lines work if you have economies of scale, i.e. if the incremental cost of production can eventually be brought below the value of the product on the open market and you can start turning a profit. The economics of planetary exploration are fundamentally different because the product is not the thing you're mass-producing. The product is data, not spacecraft. Furthermore, the hardware required to produce the most valuable data, i.e. the data you don't already have, generally needs to be different from the hardware you've already built. That's the fundamental reason planetary exploration is unlikely to benefit much from mass production any time soon.