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The 'set' method in the 'UserInstance' class becomes unchangeable because it is a predefined method of the class that is inherited from the base class 'Model'. The 'UserInstance' class is a specific instance of the 'Model' class that represents a single row in the database table. As such, it inherits all the predefined methods of the 'Model' class, including 'set'.

If you try to redefine or override the 'set' method in the 'UserInstance' class, it will no longer function properly as it is expected by other parts of the Sequelize library. This is why it becomes unchangeable and should be used as-is. However, you can still customize the behavior of 'set' by passing options to it when using it to set values in the model instance.