Well, since this is a vintage drum forum....
I think it's the sense of individuality that's missing from the scene. Back in the day, you would NEVER see a factory-made drum set from Ludwig coming out of the doors with, say, a Gibralter or a DW component included. No. Everything was Ludwig's design.
Are we talking about an "entity" such as a company like Ludwig being a builder/assembler? Or, are we talking about an artisan builder who works by lamplight out of his little gnome cottage in the Black Forest?
Are we talking about a Stradivarius (completely handmade) instrument? or, are we talking about a factory line where the shells are built in one room by one guy and then he sends it down to the next? In either sense, I suppose something could be built AND assembled by one entity...but then it becomes a matter of how "entity" is defined. Bill Ludwig didn't build the drums, but he created them....if that makes sense.
Are you asking about an individual person or an individual company as "drum builder"? My initial answer was based on the former premise.