Why isn't UE4 as moddable as UDK?

I’m questioning though why I would have to modify the source code to make a moddable core like the past versions of the engine. Has it really changed that much?
Even ARK has limits on how you can mod it, you even need to download their SDK. There’s even a bug with the map system that prevents modders from being able to remove certain assets and workarounds need to be made to fix the problem.

I’m more of referring to how the idTech games were just huge core mods and player mods would be loaded on the side after the main game “mod” loaded. Ut2k4 kind of has it more separated but even conversions like Air Buccaneers, Alien Swarm, Red Orchestra, Domain 2049 change core mechanics of the way Unreal Plays without needing source access to the main engine. From what I have seen you have to manually add in handles for mods to utilize.

What I wanted to do was setup a system like what idTech, and UT2003/2004 had where the core game is just one huge mod and then it systematically loads 3rd party mods on the side. Instead of having to make the core gameplay part of the actual engine and then having to give modders access to specific parts, including an SDK like what Ark ended up having to do. This would allow modders to have full control of as much of the game mechanics as the core game mod since they both would be considered “mods”. How would I go about doing this because as of now it looks like you make the main game with Epics tools and specifically expose certain parts to modders where instead I would like to have the entire game function as a mod so there isn’t that separation.

I’m a UE4 Noob still stuck in idTech4, UDK ways. So I probably am missing something on how to expose huge chunks of gameplay to modders.