Mod support in UE4 is such a tricky thing if you're comparing it to how Blockland handles modding. Since you have a lot more tools to work with, it becomes more necessary to more closely vet mods to make sure there's nothing malicious in them. Along with this, the way that modding works for most Unreal games usually requires using a modified UE environment.
That's why games on the older Unreal engines came with devkits that were basically just watered down UE3/UE2 environments meant for making content specifically for the game it came with. Just look at how Killing Floor 2 handles mod making!
With this said, developing mod support later into the development makes sense, it's not like you need to develop from the ground up with mod support in mind in order for it to work. You're really overestimating the difficulty of adding it.