'A good', meaning a singular possession, has been used in English since the Lindisfarne Gospels appeared in about 950 AD. In the plural, as 'goods' the usage has been with us for over 700 years, so it's pretty ancient! Today, it means movable property as in 'goods and chattels'. The idea behind the words is that it is good to have such things.
Why on earth would it ever have been 'bads'?
Now you put it so simply quiz it all makes sense. Your right how thick am i why on earth would it have been bads? Sorry for been so niave and senseless!