Most highly qualified workers loves what they do and would stand for keeping they’re output quality up. On the contrary interchangeable cheap workers have no real incentive to that. The factory’s manager is left alone in charge to balance quality versus cheapness, and the last comes with obsolescence (planned or not), which is good for business.
Because that's what people want. You can get high quality clothes for much cheaper than you could in 1816, but people prefer disposable clothes so they can change their look more often. This is just producers responding to demand.