If you look at it as more of a "composable" interface (as the sibling poster suggested), it makes a lot more sense.

No one is going to type out this one-liner from scratch, or have an easy time understanding what it means by reading it, but as it's made up of a series of smaller, more easily understood commands, in a shell or Python script it could be vastly more legible and, dare I say it, usable.

This is also the reason why there are so many frontends to ffmpeg, to simplify various specific tasks. I can't count how many one-off apps I've seen that do one thing and do it well, and just ship a full copy of ffmpeg to do that one thing. Making an actual GUI for all of this would be just... insane, really, but it's so versatile and flexible that you can basically do anything with it.