It would be pretty much a copy/paste fix in the engine. They have just opted not to.

You either misunderstood the comment you’re replying to or don’t know how the unofficial patch works or both.

99% of what the Unnofficial patch fixes have absolutely nothing to do with the engine. For example, we’ll use the Skyrim Unnoficial Patch, easily the biggest and most popular. It fixes literally nothing in the engine, it fixes certain models not having textures wrapped correctly, it fixes certain meshes or textures having small errors like clipping, it adds a new flag for a town that didn’t have a flag in the original for some reason, the absolute closest it gets to an “engine fix” is fixes for different scripts that sometimes fire incorrectly.

Literally none of these are engine issues or fixes. Sure, they definitely should’ve fixed them before releasing the game, but it’s not like these are engine issues that have somehow persisted for 20 years. They’re very small bugs with models, textures, and scripts, which are all individual game issues, not engine issues.