Anyone who had tried to make anything, especially anything original and with mechanical components knows this to be true. Even simple machines require a lot of forethought, planning, and iteration to be able to perform consistently and reliably. When we see an every day object like a bicycle we take it for granted and think that it must be obvious. But that couldn't be further from the truth.

Take even just a single part from it, like the chain for example probably represents centuries worth of technology. Each link has to be uniform to operate smoothly on a chainring or sprocket, which indicates some form of mass production.

Chains must be hardened to withstand stress, resist stretching. Soft steel would wear and deform too quickly.

Each link is in itself a complex component composed of a uniform bushing and pin shaped oblong symmetrical down the center and also quite small. It must pivot smoothly.

I doubt there are many people, even skilled people, who could make a complete bicycle from raw metal stock.