Not 100% sure, but I was under the impression that unlike Windows (?), OSX simply does not allow you to have direct hardware access. (Plus, if Parallels was accessing the hardware directly, how would you be able to run the VM in a window while still doing OSX stuff underneath?)
Either way, there's clearly a way to make the game run significantly faster on OSX, but Valve isn't doing it for some reason.
I can't speak to OSX, but on linux I know that virtualbox likes to install kernel modules into the host. If your VM does that, then it should be able to get direct hardware access.
Either way, there's clearly a way to make the game run significantly faster on OSX, but Valve isn't doing it for some reason.