Like it just becomes the case that after a while if the engine's code isn't being maintained then you wind up spending more work fixing old code then implementing new features. If tech debt becomes bad enough then new features become nigh impossible. Any attempt to add a new feature might involve having to pull the whole engine apart like having to disassemble an entire house to its foundations just to add a new window.
Bethesda's engine has been in use since Morrowind, and although it received some big update for Skyrim, and for Starfield you do start to question that maybe scrapping it to work on a new engine from the ground up (or buying a competing engine) might be the best move. As it would mean less time spent trying to fix problems, and more time spent on features