Is because they're a pane to clean.
I wash windows for a summer job when i am home from college.
Today I was washing a window to a nail salon when an older gentleman was walking past, and he hit me with a zinger.
"How's the window washing business going?"
<without waiting for a response> "You must be cleaning up!" He then walked away chuckling to himself.
We were driving to a friend's place when my brother points out a house with a rather large window and says, "That window must be a pain to wash." It took us all about 3 seconds before we all groaned.