Bethesda is notorious for buggy software releases. They could spend 10 years on something and it would likely have tons of bugs still.
0: I recall reading somewhere that the game uses a really old version of Bethesda's proprietary engine too - but only for physics.
See the change log here: https://www.afkmods.com/Unofficial%20Skyrim%20Special%20Edit...
Only a very small amount of the issues fixed there where integrated into the official patch releases.