[Animats]

t's trying to contain software which wasn't written to a strict boundary, to a strict boundary.

Exactly. That's why NSA, decades ago, wrote SELinux. It wasn't intended to be a security measure. It was intended to encourage development of user-space software which lived within strict security limits.

That never happened. The desire for loopholes ("must phone home", etc.) beat security restrictions. All a single-player game really needs is read access to its own assets, input from user input devices, output to graphics hardware and sound, and the ability to write in its own preferences/save directory. Try to find a commercial game which will run under such restrictions.