The issue there is the amount of memory usage, large scenes, especially with foliage will use a ton of memory. While playing a game it will only load the things that need to be visible, so things like foliage are set up so that you only see the stuff that’s closest to you. For building lighting it has to load all assets and have enough memory to have all lightmaps in memory at the same time.
Big open world games can’t do that since it’s too much data, so they use some type of dynamic solution–the Kite demo for example uses entirely dynamic lighting so it doesn’t require building the lighting beforehand. The problem though with that is that you don’t get as nice of lighting, mainly because it lacks the nice bounce lighting (Global Illumination) that you can get with the baked lighting. A lot of games use some kind of simplified version of dynamic GI, but moving entirely to a dynamic GI system is the dream. The issue is getting good enough quality while also getting a good framerate. There’s lots of different systems out there, for example with UE4 you can use something called Light Propagation Volumes (LPV) but it is a bit limited in features and doesn’t have great performance and not very good quality either, but for some games it provides that little bit of bounce lighting that helps make a game more realistic. There’s also VXGI from Nvidia, which is available in a separate branch of the UE4 source if you want to try it out, it’s much more complete but isn’t super fast. Cryengine has SVOTI, but it’s still got some of the performance issues and other problems that dynamic GI systems have like light leaking.
Unity and Frostbite use a lighting system called Enlighten, which allows for high quality dynamic GI however it has a lot of drawbacks–it has to be built ahead of time so you still have the issues of memory and having to spend hours building a level (usually also slower than what you can do with the UE4 baked lighting system). And after that the only thing that’s actually dynamic is the lights, which you can move and change just fine, but no moving meshes can have dynamic GI and anything that doesn’t exist in the level at the time of building doesn’t get dynamic GI either.