why can’t any of the X/S series optimized games run off hard drives, it makes zero sense to...

Yes you can. The expansion card is for Series S/X optimized games. So you should be able to run any game from them as opposed to regular external hard drives which cannot run Series S/X optimized games.

It's only on the Series S/X that they can't be played on a regular hard drive. The hard drive is too slow to run the games properly. The One is fully capable of running all games from the internal or any external hard drive. The One wouldn't have been playing optimized games at the full S/X optimization due to hardware limitations.

If he has a Series S/X and is trying to play a Series S/X optimized game, it will only run on the internal drive or on an expansion card made for the Series S/X. It won't run on a regular external hard drive or SSD.

For Series S/X optimized games, you can only play them off the internal drive or one of the expansion cards that were created specifically for the console. They will not run off of a regular hard drive or SSD.

Any games that are Series S/X optimized can only run on the internal hard drive or the Seagate Expansion card. The external hard drive you are using can only store them.