I call this the "windscreen wiper effect".
No matter how hard it rains it will never clean the dirt off the glass on your car, but run a wiper over it to dislodge the dirt and it will come up clean, its the same with the brush.
A pressure washer will only clean off the larger dirt but wont touch the stains and dirty film that builds up. You need to dislodge it and get it into solution before it can be rinsed away.
I sometimes use a pressure washer on facias to get into all the dificult areas and remove the heavy stuff if its really bad, but then polish it up with wfp.