Why there is no way to add variable declaration inside rules?

[vec715]

Writing rules always limited to working only with the provided set of structures and their fields. When some function should accept some arguments we need to define them directly e.g.:

rule ... {
    when
        ...
    then
       fn.SendAlert("title", "body", "receiver@gmail.com", fn.CalculateSomethingX(), fn.CalculateSomethingY());
}

But it would be much much easier to read & maintain the rules if we can have a variable declaration:

rule ... {
    when
        ...
    then
       var x = fn.CalculateSomethingX(...);
       var y = fn.CalculateSomthingY(...);
       var receivers = ["foo@gmail.com", "bar@gmail.com"];
       var body = "very long long body here";
       fn.SendAlert("title", body, receivers, x, y);
}

Why this feature is unavailable? I wonder what type of limitations we have so there is no such a way to declare variables inside the rule?