Perhaps it’s the limit on total resources used in a settlement. There is a mod to change that.
They had to draw the line somewhere I guess, otherwise people would be building megatowers and crashing for lack of VRAM...then screaming at Beth because of it.
I’m waiting to mod until after the Creation Kit is out (and I finish the game at least once stock.
Come to think of it, Skyrim had a couple cap remover mods as well. The 4Gig mod that opened up the useable memory cap and the open world mod that made the insides of the big holds part of the greater world - no load screens/seemeless movement from outside to inside the walls.