Cannot Use Variables Inside A Case In Switch Construct?

If I have an integer variable like int a=9 then in the switch case If i write

switch(a) {
case 4+a: System.out.println("hii");
}

Then why is this statement a compile-time error that variables cannot be used inside a case statement why does the compiler not subtitutes the values in place of the variables.

So basically what problem it creates for which the language developers did not include it as a proper syntax,is there any reason behimd this because of jump table?