[geoschmo]

This wasn't always this way. It was added in a patch, I think 1.60, at the same time the restriction against adding a colony comp to a ship was added. It was done to nerf the retrofit tactic a bit. Otherwise you could build cheap empty base hulls over your homeworld and retrofit them to space yards and exponentially expand your construction capacity. You can still do the same thing, but you have to spend some points to your empire setup to get a construction rate that will allow you to build a space yard base in one turn.