I can. "License: Proprietary freeware EULA" on
http://en.wikipedia.org/wiki/Flash_PlayerAnd the argument that anyone is free to re-implement is just as valid for anything. The fact of the matter is that thousands of man-years have gone into building Flash Player, and replicating them is far from a straightforward task. The spec is huge, and there are millions of .swf files to which an alternative implementation needs to be compatible. It's a bit like suggesting someone re-implement Windows if they don't like Microsoft's version of it. It's open, right? Anyone can look up the ABI for user space executables, kernel drivers, etc., so how hard could it be? ReactOS (or Wine) hard, as it turns out.
Note also that for the supposedly open components, Adobe controls them so tightly, they are only "look but don't touch" open. Openness is only useful insofar as you can actually do something with the information or code. They maintain this control precisely because they have an iron grip on the player. There is no point for a third party to extend the Flash spec or compiler, because they can't realistically build an implementation that actually runs code obeying the forked spec. It's as ridiculous as creating a driver to run on a hypothetical version of Windows. Yes, you can do it, but it's also completely pointless.