I've examined this problem and been keeping track of it for nearly two decades now, keeping an eye out for legal resolution of it, and to the best of my knowledge there is still bupkis on this front legally. Even before we consider EULAs, it is legally quite clear that they could not take the two copyrighted products Fallout 3 and Fallout 4 and distribute the resulting new work. (And what EULA have you ever seen for a AAA game that would explicitly permit that?) It is completely legally unclear whether it is OK to release a tool that the end-user nominally uses to do the work themselves. On the one hand, there are certain arguments that this should be OK; I'm sure in the next few hours a few dozen of these will pour in. (Though I will say the most obvious error they make in general is to grossly overstate the degree of ownership the end-user has over the content, and, again, that's even before considering EULA clauses.)
On the other hand, if these arguments are accepted, then it allows a clear path to completely circumvent the aforementioned clear legal result that the end product could not be distributed. This is generally frowned on by the common law legal system, cynical comments to the contrary. It is also in a very real sense arguable that the user, who is putatively putting these two works together, can't really be said to be doing so when the sum total of their contribution is "the user clicked a button"; how can they be said to be responsible? It's pretty obvious that button-clicker can't be said to hold any copyright rights over the result, because "clicked a button" certainly doesn't raise to the level of creative input. It can be easily argued that in a legal sense the program is still the responsibility of the original combinind developer, and that the developer is still as responsible for the output as if they were directly distributing the output, what with the end results being (potentially at least) literally bit-for-bit identical.
I remain unaware of even the slightest hint of legal guidance in this area. And please note I'm not in this message trying to advocate either way; I'm simply discussing the situation. (I have opinions, but they are incomprehensible until you first understand that there is a problem at all.)