They tend to do that by tacking on new jank without removing (much) of the old stuff, though, presumably because they have base assets and scripts that they're constantly re-using. Or, differently put: As long as Papyrus will still be in the thing I seriously doubt they're giving any thought to technical debt. Already in Skyrim people rather used the UI to script stuff (because that's Flash and ActionScript is at least remotely sane and fast) but ultimately it's SKSE (that is, native dlls) for anything that isn't a lag fest.

It's not so much that CreationEngine is easy to mod, it's that it's what a gigantic community of modders are used to and have developed tooling for (you can get by with little to no use of CreationKit which is an abomination all on its own). Stockholm Syndrome at its finest or we'd have seen much more content for RedEngine which is far technically superior (and yet CDPR is abandoning it for Unreal).