1. ffmpeg exposes all of its options through the CLI, and there are a lot of options. So it's probably always going to be completely undiscoverable. It really needs a GUI to be usable, but that's a project in itself (I guess the project is Handbrake).
2. They probably didn't put a lot of work into the UX of the CLI since it's an open source project.
3. Backwards compatibility.
I don't buy this. A TUI is completely possible, e.g. LazyGit, Htop, etc, countless tools indicate you can get pane-based UIs going in the terminal; the FFmpeg team has simply never made such a thing a priority.
But even without a TUI, the most basic use-cases are well-known after over 15+ years of existence, simple prompt-based wizards, i.e. "git add -p", should be offered; again, a matter of priority rather than being intractable.
A year ago or so someone posted a subscription service they made for composing the pipeline graph visually
Sure. A TUI is basically a poor man's GUI.
> simple prompt-based wizards, i.e. "git add -p", should be offered
I agree. Wouldn't be hard. I think you may as well use Handbrake though.