Every feature seems simple by itself, but considering the overall scope and the maintenance effort, you will need dozends of people working full time on this. (I worked at such a very for profit media tech company)

The scope gets bloated and the app unfocused. Most people wouldn’t read eBooks or comics from a big screen interface for example, but use a dedicated app for that. Keeping a cinema app primarily focused on video means it is leaner and meaner for the majority use case.

Because people want a massive piece of software for free and "expect it". I mean we would all love to have what you typed out there but it would be a massive effort and not likely to happen anytime soon. Jellyfin and Calibre do a lot of what you have there but probably not even the majority of it and certainly not flawlessly