How does Haskell hide data constructor?

I explored System.Random.StdGen and saw this code in the source.

data StdGen = StdGen Int32 Int32

It seems the module export StdGen too.

module System.Random (

    RandomGen(next, split, genRange)

    , StdGen

    ...

However, why can't I do this in my code, say like,

Prelude System.Random> StdGen 1 2

Not in scope: data constructor `System.Random.StdGen'**

In the other hand, I can do this,

module F (Foo) where

    import GHC.Int

    data Foo = Foo GHC.Int.Int32 GHC.Int.Int32 deriving (Show)

and

Prelude> Foo 1 2

Foo 1 2

Would someone please kindly tell me how actually this data constructor is hidden?