Same reason we don't have periscope on the Ar234.
The reason there is no 'periscope view' on the 234 isn't due to lack of pictures and is in no way similar to the ventral gun on the A-20.
I have about 10 pictures of the periscope I found on the web. I know many more in other books.
The reason there is no periscope on the 234 is the difficulty it would require in getting it to work correctly. This is the same reason we might never see a Tu-2 or 410. It isn't a graphic problem per se but a problem of proper function.
Besides in AH they don't use 'accurate cockpits' and while in the 'periscope view' it would look about the same as a tanks main gun view.
The periscope in the 234 served several functions, including a bomb site for shallow angle attacks. I had posted many times on the forum how it works and even some pictures.
So the 2 are completely different.