Why is this? This is a very necessary feature in my opinion. I have spent ages getting a graphic ready and my artist would also typically create PNG. It's a great format for smooth images. Alpha channels are not the enemy. If an alpha channel is right at the edge of the object (where the alterable transparency takes place), it does not affect the object itself and the alpha channel could just be ignored. GIF is archaic and few modern devs like it. Colours are limited, file sizes are higher and transparency looks horrible and jaggy.
This should have been expressed since I've spent hours investigating why my objects were falling and now I've discovered why. Whilst it might be true that you can layer a second GIF active under the PNG, this isn't a smart approach and creates unnecessary bloat. This is 2014. I'm sorry to rant, but the physics are really outdated without PNG support.