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