DC adapters: why so few amps?
The components that make up DC adaptors (inductors, transistors, capacitors, diodes, ect) are all rated for a certain current and/or power dissipation. Components that can handle 1000A vs. components that can handle 5A are orders of magnitude apart in cost, size, and availability. 

For an example let's look at an inductor that could be used in a 1000A supply vs. a 5A supply. 

Price: An inductor that can do 5A is $0.17  on digikey, an inductor that can do 200A is $400.

Size: The 5A inductor is 5mmx5mm and the 200A inductor is 190mmx190mm.

Availability: Digikey stocks well over 5,000 different inductors that can handle 5A. It didn't even have anything rated for more than 200A. It stocks only 7 that can do more than 100A.

Now repeat this experiment for all the components found in a common wall adaptor and you'll quickly get to the answer of your question.

To summarize: If you had two devices that needed 5A and 6A respectively would you rather buy something that costs in the thousands of dollar range and is larger than your bathtub so you could use it on both, or would you rather buy two palm sized adaptors for $30?