I don't know a great deal about ruby, but I'd wager that the way it's designed would make static type checking essentially impossible. And you probably understand this, but still. Dynamic languages allow fundamentally unsound types, for example:

    def foo(x):
        print x
        return foo

This function has no computable type (in a standard system anyway); it's "a -> a -> a -> ...". However it's perfectly obvious what it does, and conceivable that it or similar functions might exist and be useful in actual code. Dynamic languages allow behaviors which are impossible in statically typed languages (properties created at runtime are another example).

Of course, one could use gradual typing to get around some of this.