Because the GK can legally handle in the penalty area. And a thrown object is considered an "extension of the arm".
So... you can't actually punish the GK for handling in the penalty area, nor can you give misconduct for it.
What you can do is penalize the act that brings disrespect to the game (the throwing of the object), stop the game, caution the GK for that, and give an IFK at the point of contact (where the offence actually takes place, oddly).
Now, this can't be DOGSO, because you're not stopping the game for the free kick... but for the caution. And thus that caution can't magically grow into a dismissal.
In short... it's a weird circumvention, but it exists. And who knows, the big rewrite might alter that.