why do flies never leave thru an open window but insist on bashing their heads against an adjacent open windo

Answer 1

Most likely because it isn't natural. They are unable to understand the strength of the window, the probably just see that there's nature a bit further on, and something is blocking their path. With no understanding of the window's strengths, they will simply keep trying to get outside until they succeed or find another path. So in a sense, they can't tell the difference between opened and closed windows, so they take a 50/50 chance. You probably only ever notice the times they choose the closed window, because the sound of it becomes annoying.

Answer 2

Perhaps the ones that are trapped against the glass are more obvious than those which simply fly out through the open window?