The reason is that counterfeit and off brand chargers are poorly made, and that means they put your device (and your house) at risk. Many ignore safety standards completely, but they also just don't charge your devices that well because they don't push the amount of power they're supposed to. This means you're spending more time with these devices plugged in, which causes a larger risk to your safety, as well as your device.
That's about it. If you pick the right charger, your devices will charge as quickly as they're supposed and your house won't burn down.