The problem with UE4 in film/animation is about the the place it should occupy in the pipeline. Being a game engine at its core, it's fundamentally designed to ingest models and render images. So its natural place is at the end of the pipeline. There's no really good roundtrip capabilties that let you e.g. export a scene that was created in UE for use in other tools. So despite the interactivity that would let you e.g. block out a scene with awesome real time feedback, there's no good way to get the data out into a different animation or lighting tool for final touchups before it goes to an offline renderer. This is why some big animiation studios have invested a lot of effort into in-house realtime preview renderers that attach to their tool pipelines with less friction.
That's an excellent point about why they would rather write their own in-house software. Of course they then get saddled with having to support all of that software and may struggle to do so, which is another problem in itself.
