I believe it to be a capacity issue... most people willing to go to the effort of upgrading to an external amp (when replacing an integrated head unit) or replacing a factory amp are looking for something better. Improved sound quality and/or volume are going to require more power out of the amp, which is going to require greater power draw than what the factory wiring accounted for. Kind of like upgrading your fuel system... you want to get more power out of an engine, you'll need greater fuel delivery, which will sometimes require larger fuel lines and a bigger pump. Even if you're not using all that power, you have to make concessions to be able to deliver it.
I don't imagine that there's a huge demand for factory stereo-level power output for aftermarket amps that would support the aftermarket going to the trouble of designing and manufacturing equipment to those specs. Let's be honest, probably 90% of factory systems are value engineered to only supply the bare minimum quality that a buyer will accept.
