Video players are among the most difficult of applications to sandbox. The codecs involved may be in-app software or hardware reached via one of a dozen abstraction APIs. The same is true for audio and frame buffer access. There is a huge amount of code that needs holes poked in the sandbox walls to function.

And realistically once you do that you have another component out there with all that complexity and permissions to exploit. That's exactly what happened with Android. The apps have a clean sandbox, so all the exploits target the mediaserver process instead.