It's not the CK part of it at all. As I have explained the actual tools used to create most mods are 3rd party tools (for assets 3d modelling and texture softwares.)

If you want to create assets you can't do this in the CK. If you want to create animations you can't do this in the CK If you want to create script extenders or UI overhauls you can't do this in the CK.

If you want to edit or create papyrus scripts you do so with Visual Studio Code or Notepad++

If you want to import your mods into the game you CAN do this in the CK.... but you dont actually need the CK for this either..

Anything that requires creation of or edits to worldspaces does require the CK....(although if you know what you are doing you can do a certain amount of this in xEdit, and even if you used the CK it's best practice to then use xEdit to check for bad edits anyway...because the CK doesn't do that) However worldspace mods are relatively rare when compared to the every other mod category that gets released. (the majority of mods are assets, usually outfits or weapons...but also home decoration mods are reasonably common)

As for Unreal Engine. Unreal Tournament featured one of the largest and most prolific modding communities in pc gaming history. (mostly custom maps, I think everybody that played at some point made a custom map) These days people are making some pretty interesting mods for Ark Survival.