Suggestion that humans don't use symbols like em dash is based on the possible difficulty to use them. The reason is probably because many never used / heard of the Compose key. It's not hard to do it once you learn about it.
I don't think it's because they are mechanically difficult to use as you imply, but rather that they are subtler or more nuanced symbols than most people know how to use.
A lot of people are comfortable using the dot, the comma, and maybe exclamation marks.
AI-speech seems to strive for more formal writing by default.