Because when you have the preformed parts, you just go over and snap them together. When you have to print the parts, you warm up the printer, download the files, print the parts, fiddle with the printer, print the parts again and snap them together.
Perhaps as an R&D setup, this makes sense - if you are trying to develop different frames / gizmos / attachments to the UAVs to fit various mission requirements. In a shooting war, not so much.
"Sailor, we want an attack drone."
"But sir, if I just adjust this part some more we can make it go faster"
"Sailor, print out the goddamned drone."
"But sir, I can make it so it has LED lights!"