i think the easiest way to explain this:
You know how some software is more geared towards arch viz, and some towards mograph, and some towards character animations. They each put a different emphasis on various things and tools. Arch viz software probably won't bother too much with developing hair tools, just enough for grass and carpets. Mograph software isn't that concerned with character animation, just enough for a guy pointing/holding a product, maybe some 10 second animations.. Character animation software doesn't really care that much about cloning objects or interactive text objects. But all these things are still pretty close to each other and overlap. Since they all focus on offline rendering. So the interactivity takes priority during creation.
Unreal is a game engine, the end product ABSOLUTELY must be interactive that is the highest priority. So they sacrifice interactivity during production. And the tools they develop are largely geared towards creating games, not movies/animations. They are objeviously moving towards making movies, and are being used on projects, but at least with unreal 4, you still feel at every moment who this tool was created for.
It's kinda like using a sledge hammer to hammer in nails. It works, and if you line up a whole bunch of nails, you might be able to nail them all in at the same time, but it's still very awkward :))
Or boiling a 10 liter pot of water instead of using an electric kettle. It does boil a lot of water at once, but then it's kinda awkward to pour it into small cups, and it doesn't shut off automatically etc..
it's just lots of small little things which add up during your usage experience.