Why do clothing manufacturers from developed nations use sweatshops in developing nations?

They naturally seek to reduce the cost of manufacturing by contracting out manufacture competitively on a global basis.

Local companies in developing nations win these contracts competitively because they employ inexpensive labor.

People put too much blame for sweatshops on the developed nation companies, it is up to the governments of developing nations to regulate their labour laws to ensure people working for local companies get a fair wage and a safe working environment.