Part of that is the ability to focus on improving on all of the work that already exists and the other part is being able to unilaterally make decisions while being emotionally invested.

Developers are often spending the majority of the time making sure whatever gameplay exists passes testing without breaking the overall compromise vision that is limited by time and money for deadlines. Companies don't allow enough time for polish and frequently have decisions made based on the added cost of labor and testing tha mods don't have since mods having bugs is an acceptable situation for people while a company putting out bugs is generally met with hostility. Different levels of standards and costs have a huge impact on why many mods vastly improve the game in ways that don't fit into the game development process.

Plus a lot of mods are focused on a specific part and don't appeal to the playerbase as a whole.