Paul Dushkind writes to ask…
I have to ask, why do screenwriters type in Courier? Does it make it easier to count, if you're paid by the word? In direct mail advertising, we used to use Courier or other "Typewriter" fonts, because a direct response letter is more compelling if it looks like a personal letter, which in those days might have been composed on a typewriter. That doesn't apply now.
Well, I've never heard of a screenwriter being paid by the word so it's not that. Beyond that, I'm speculating here but I suspect a lot of it is simple tradition. Courier is what almost everyone uses. If you started getting fancy with your font, someone's going to complain it's harder to read or to estimate time…but no one can complain about Courier.
Directors and production assistants learn to estimate timing based on your traditional page set in Courier. If you use something else, they have to pause and wonder if ten of your pages will turn into the same amount of screen time as ten pages in Courier. Also of course, agents tell new writers to make their scripts look as much like produced scripts as possible. If you type in something odd, it can seem amateurish to some. If you were writing a "spec" episode of 30 Rock to submit for consideration, why would you not want your script to look like a real, ready-to-shoot script for that series? So that kind of thinking has probably helped institutionalize Courier as the professional font.
Does anyone have another theory?