An alloy adds strength, but it can also stretch how much the manufacturer can get from a given amount of copper.
You just add other cheaper metals. That's where the cheapness comes in.
That's where the quality control comes in. If you buy off-brand crap on ebay or wherever, then you don't know what you're getting. A name brand has a reputation to protect. So they will run QC checks on the stuff coming off the boat. If it turns out to be not up to standard, then it will be refused and returned. Or the supplier will be terminated.