I think it has to do with the Ohm rating of the stock system. To get the marginal OEM amp to power all those speakers they have a very low resistance. i.e. low ohm rating. Maybe 1/2 to 2 ohms.
.
Most (if not all) quality speakers have 4 ohm ratings. Meaning to change out the speakers to some quality sound you may as well chuck the amp with the cheesy speakers. My home tower speakers are rated at 6 ohms.
.
This is why the stock speakers suck. They can't handle, or rather the signal from the amp, can't handle lows very much (requires more power) and the high sound screechie!!