Even people building up bikes from a frame/fork are not getting a custom bike. They are using off the shelf parts to build their ride. I built up my LHT from parts...their isn't an identical Trucker on the planet, but the only part on the bike that is actually custom is the mud flap I made from a milk jug.

Mixing and matching production parts is a cost effective way to get a specific set of performance and aesthetic results with minimal risk. I can use some Shimano Dura Ace barcons with SRAM Jag Wire cables and a XT reverse rise rear derailleur to get my LHT to shift with little risk, reasonable cost and it meets my preferences, but it's not really custom. Not one of those parts was made for me to my specs. The bike industry is just standardized enough we can mix and match a fair bit.

If the parts we use on our bikes weren't made in the thousands to a common standard of functionality they would just be way too expensive to use and wouldn't be particularly versatile. Although we treat custom with an implied assumption that it will be better than production I don't think that's necessarily the case. Production parts & frames have lots of things going for them.

If you have me the option of a production Riv and a custom bike of equal value I'd likely choose the Riv because I'd know exactly what I was getting. Both in terms of function and aesthetics.