I'm guessing because, for what most people use them for, a sub $300 firearm is a lot more practical financially then a $500+ that will do the same thing. Sure everybody wants the best. The sad reality is that only a few can afford it. The rest of us have two choices: 1. Do without. 2. Buy generic and hope it's good enough.
Now if you are looking for collector or heirloom value, you wouldn't be looking at "Made in China" in the first place.
I just bought a nice used Wingmaster for $229. My 870 Express, new, cost around $300.