It really comes down to cost and space. The 5 watters do their job good enough to justify their existence in the space they are given. Think of an extension cord... The thicker the wire the higher the cost. The thinner ones get warm but do not catch fire. Everything still works fine. Same goes with your resistors. They can operate then at the highest thermal specs and save some money. Save a dollar on a million amps and what do you end up with?
