this is not true in my experience. prefab kits of all sizes (from sheds to houses to barns, like were once possible to order from a Sears catalog) have worse tolerances than a carpenter working on site. you can measure 3 times and cut perfectly, and still end up with a few mm gap (or sometimes worse) after tiny errors accumulate as you assemble piece after piece. it _requires_ measuring as you go and cutting on site to handle this small amount of drift and to really produce something of high quality. it doesn't come in a box

Correct about large scale kits. I had meant to head off the fact that preassembled pieces like windows have improved a lot, things that used to be assembled on-site but are now delivered as a unit or small kit.