Didn't say it was easy, it's easy to learn enough to hold a round down even when you are a newbie. Some don't learn properly or wont take advice. They see it as a money printing machine and cut corners, OR they go in too cheap and get P'd off with doing the job and then because of these reasons, do a crap job and lose custies.
I agree with what you say about a WFP job for 20 or a trad job at 20. WFP is faster, but I wouldn't set my price by the method of cleaning. If someone will pay 20 for a window cleaner, then I take 20, whatever method I use, they are ready to pay 20. BUT, if I have a custy ready to pay 20, why should I do it cheaper because I'm WFP?
Some guys are charging hourly rate, so logically, if they are WFP, they are charging less than the custy is ready to pay.