The PS5's internal SSD is specially designed to work in tandem with the console's CPU. There's a lot of technobabble involved, but PS5 games cannot be run from external storage because external drives, even solid-state ones, are too slow. That being said, PS5 games can be stored on an external drive. However, you cannot play them from the external drive. The games must be transferred to the faster internal storage of the console to play them.