Properties are just sugar-coating syntax for a getX() and setX() method. It looks and acts like a field, but it's really just two methods. The reason why the auto-property was added was to avoid the repetition of having to create a field and creating a standard getter and setter for the property, and to make it much simpler to allow changing the implementation without changing the interface.
The reason they can't be accessed by reference if they are a value type is because value types are generally on the stack and because you're just calling a method. The getter in the property has to be called and the returned value has to be pushed on the stack before it can be referenced.
