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Probably weight. Every ounce you add to the weight of a plane is more thrust you need to keep the thing in the air or less munitions you can carry.
The question I would ask is “Why were these things or something like them, not included in the Space Shuttles?”
Same answer, but astronauts are a bit more expensive than pilots to “create” (training, experience, etc.) I would expect them to be worth the payload loss that including a set of these would have involved. Of course you would also need to look at the near total redesign on the interior of the crew section of the orbiter, but that is something that could have been dealt with in the early planning stages.