Because they are dirt cheap. Dirt being the central word.
I personally boycott Made in China goods as far as possible.
Unfortunately it becomes harder and harder especially with plastic and electronical goods. I try to stick to Made in Taiwan, Korea or Philipines then.
I can smell the arguments now. They always start off with the condescending, whiny, liberal-rodent like phrase:
"Well, ya see, Mr. Heep..."