You gotta think 1975, or so. These days we know of online shopping and everything at low prices, well that was not the case 42 years ago. New aftermarket parts were expensive, and seldom worked correctly.

So a guy has a wrecked car, neighbor with a wrecked car, junkyard guy that gives good deals, etc. get a carb from a factory 383, it will bolt on, jetting is right for the size of the engine, kickdown will hook right up, factory choke will function for winter driving, etc.

Myself I always get a factory Chrysler carb when putting together a driver type car, they work and hook up correctly.