By all accounts it's been also maintained terribly over the years.

i think that is a bit unfair to bethesda's engine. all those other engines have achieved their scalability at the cost of extensibility and easy to work with game systems. this manifests most visibly in how mod support works for bethesda games versus games built on other more "optimized" engines, but it affects the core game design as well.

even if id software released their internal tooling to the public, it wouldn't be all that useful for making the kinds of mods people make for bethesda games, because their engine isn't built for all the systems-driven game design that bethesda's is. that moddability is born out of how bethesda has designed their engine, the gameplay systems they built in it, and the tooling that supports all of this.

it's truly insane just what you can do with bethesda's engine with relatively little work. and it shows when you compare to games that try to imitate their game design on other engines. the outer worlds felt really static compared to fallout new vegas and skyrim, because it was missing so many of these systems.

bethesda games have a lot of problems, but ditching their engine for something like unreal or id tech would most likely destroy most of what makes their games unique.