Suppose everyone communicates via scribes, who write in ancient Egyptian hieroglyphs, and Microsoft is the only company who trains them.
Now there do exist some talented linguists who can figure out approximately what they’re writing. That’s what the Rosetta Stone was about, except in this analogy there’s no Demotic or Greek alongside, only the hieroglyphs and a scribe to tell you what they mean in English. Unfortunately, after they send the scribes away, the linguists can never be sure that they’ve got it right, especially since Microsoft can change the language as they like simply by changing the curriculum. As soon as new scribes start writing in this new language, the old scribes will stop being able to interpret some messages you receive because they don’t know about the changes in the language.
So to be sure the message gets through, you have to hire a scribe. When Microsoft changes their curriculum, you’ll eventually have to hire a new scribe, who will probably also demand a new house to live in. If you send me an important document in Egyptian, you assume I’m willing to do all that. You’re propping up a powerful monopoly (the scribes can after all twist your message or send a copy to Microsoft without you knowing), and assume that I’m willing to do the same.
If instead we communicate in English, which everyone knows, then neither of us has to hire a scribe. Even if you don’t know English, there are lots of scribes available from many schools, so you don’t have to put up with a scribe from one particular school if you don’t like him.
(Background: I just booked a place on Swansea uni’s postgraduate open day, and got a Word document in return, which has the timetable in it (at least that’s what the sender said). I replied asking for a plain text or PDF document, and a shorter version of the above by way of explanation. The initial booking email also had my signature in it which says to avoid sending me Word documents – unfortunately it seems like the recipient completely ignored it.)