What factor limits power supply output power?
2.4kW at 12v would mean 200A.

There is no fundamental problem in principle with making a power supply like that. However, you've seen the size of cables used in your car to take 200A between battery and starter motor. The same would go for the diodes and transformer used in the supply. It would make for a very expensive piece of kit. 

If you only need 1A to power your router, a 1A supply, with much smaller transformer and diodes, is rather cheaper than a 200A one.