Compiler gives error when struct is not initialized and if we try to access the property but not with variable

I have one observation about struct. When I declare a property in Struct and if I don't initialize the Struct then it gives me the below error - "Use of unassigned local variable empStruct"

PSeduo Code-

struct EmpStruct
{
    private int firstNumber;
    public int FirstNumber
    {
        get { return firstNumber; }
        set { firstNumber = value; }
    }

    public int SecondNumber; 

}

Program.cs-

EmpStruct empStruct;
empStruct.FirstNumber = 5;

But when I declare public variable then the above code works.

EmpStruct empStruct;
empStruct.SecondNumber;

So my question is why compiler not gives error when i try to access variable.(In case of Class it will give the error).