Some of the people who produce videos for a living require vastly different tools than someone who needs to trim the edges of a short home video clip, so the the UI and UX has to be different, otherwise these people won't be able to effectively do their job.

For writing, everyone pretty much does it the same way. You sit down, you enter characters with a keyboard, and sometimes to remove/edit stuff. Of course, there are professional tools for people who write stories for a living, that helps you keep track of arcs, characters, environments and so on, and many professionals do use them.

So while it looks like "Ah, Word actually works for everyone, why can't we do the same for video?" there are still professionals who need tools specifically for "writing stories" or "writing screenplays", and same in other areas :)