[MRtecno98]

A human leg isn't designed to be repaired from the outside, it doesn't need replacement parts, it doesn't need external maintenance. It's designed around the maintenance and repair procedures of the human body which are able to fix it in situ and produce new parts on the fly. That allows it to have more freedom of design than a mechanical part that needs to be made from standardized parts in order to be serviced and manufactured.

The limitation is not cognitive, it's a matter of efficiency. We could, if we wanted to, make a lamp with all custom parts with the maximum efficiency possible in the design, but those efficiency gains would be at the cost of extremely difficult manufacturing. It's a tradeoff that the human body can make because it needs to produce only one (or two) legs, while us humans mass manufacture the stuff we make.