Gaming companies often are too cheap to pay their own developers the cost to produce high quality results so they cut corners. Modded games are usually modified by teams of people putting in work they’re passionate about, often for free or very little pay but they don’t have to worry about some asshole manager breathing down their neck to finish an unrealistic deadline.

It’s the same reason why often, open-source software is just as good if not better than the proprietary garbage produced by corporations for profit.