Problem is how fast it would run out of water. Running "dry" damages pressure washers.
Even the smallest ones use about 5l a minute and nobody would buy one with just a 5l tank on and hence one minutes use.
Also because you want a pressure washer with it's own water supply, the tendancy is for it have a diesel or petrol engine so you are not dependant on a power source either so it can be used anywhere.
Motorised pressure washers are much more expensive and tend to be bigger/more industrial and not designed for home use.
The Karcher is probably the closest you will get but at £2000 it ain't cheap!
Surely cheaper to have a hose pipe?????