ECS has really nothing to do with this. ECS is just a specific way to store the internal state of a program, fundamentally no different from other data structures.

Also, a good game is far more than just text and images and current "AI" can't even generate those individually. A game needs significant thought put into things like game loops, story arcs, balancing,... that are non-obvious when existing games would just be training data. Not to mention that using an existing game as training data is both non-trivial and also we just don't have the vast amounts of them that current systems seem to need to produce anything even half-way decent.