I believe that's mostly because it doesn't work reliably. Doctors, lawyers, architects etc have a somewhat limited professional vocabulary and often say the same things, so voice recognition works pretty well for them. But when you write a random message, you have a much broader range of topics, and dictation that fails quickly makes the whole thing change from an improvement to an ordeal. "No, not 'or deal'. delete word. delete word. delete word. O-R-D-E-A-L. Yes, that's it. No, don't write that, sigh".