Why a new Amiga game in 2013?
Because someone wants to make one and gets some form of satisfaction out of it.
Whether or not anyone will want to play it is down to whether or not that game is shit or not.
A shit game is a shit game regardless of what platform it's on.
I think most platforms (Amiga included) have PLENTY of shit games.
So, if someone wants to make a non-shit game go for it! I love non-shit games.