It all goes back to the answer key. A matching decompilation is graded by the compiler, so a wrong guess costs nothing. Graphics, netcode, physics or a user interface have no such referee, so code that looks right but isn't can go unnoticed for years.
And even when AI code does match, it's often ugly and hard to follow. Byte-perfect code nobody can read doesn't help much if the goal is to understand, maintain or mod a game.
That's the bigger lesson for software developers, too. AI has made reading and translating old code dramatically cheaper. It hasn't made judgment, design or long-term maintenance any cheaper.