Did you not watch the episode where it debuted?
Those 5 Buster Machines weren't really meant to be combined and it was more of a "We need power now and we have no time" thing.They also had to configure all of them with the software to combine in 24 hours so they could quickly rescue the citizens, which isn't enough time to make another software that could automatically combine them.
I guess you could say that they were originally meant to combine because of the blueprints being from 13 years ago, but remember, Masato was using limited resources from Hyperspace, so he probably couldn't find the materials or develop the software needed for that.