Because the developers decided that for this iteration of the series it was better if you had to build the base improvement and then each upgrade of it, rather than going with the GCII model where you could build the fully-upgraded improvement immediately or choose to save some manufacturing costs and get a lesser improvement sooner.

One justification given for this decision is that the planet output ramps up more rapidly if the improvements are built in this fashion, which is true within limits but also becomes false once the planet's output becomes large enough that the difference in improvement manufacturing costs becomes largely immaterial.