And it is a good question. There is more to building a bike than just the assembly of parts. It needs to start, stop, turn and go over bumps without leaving parts on the ground. Often times a build starts and one of those capabilities is sacrificed in the name of style. And that ends the functionality of the bike. Fine if you admit you are building a show bike. But if you are saying it is a bike to be ridden, well to hen there are some issues.
Back in my magazine days we used to get the very first bikes aftermarket manufacturers built...and most of them saw at least one bike being sent back to be fixed before we tested it. They accepted that the quality and safety of the bikes rider were important and they fixed the bikes so we could do a test. Conversely, a custom bike built too occupy show space and garage space could be built to not ride...but if they couldn't ride they couldn't be included in the magazine..and we were ok with that
I know many more quality builders than I do hacks, but they are out there. If you select a builder, ask him if all the bikes he builds can start, stop, corner and go over bumps...might be an interesting conversation after that