I'm sure he said he could never play for another English team.
My guess is that he wants to live in England for his family. He has four (?) daughters that all grew up in/around Liverpool. It must be a brutal culture shock for them and his wife to be moved to the middle ages.
From what I understand Klopp wanted to extend Firmino's contract so I expect there was a contract offer on the table from FSG. Still, his agent will have made a few £mn by moving Bobby to and fro Saudi Arabia so that's the most important thing.
Klopp has shown with the likes of Gotze that once he's crossed he doesn't go back. Bobby will have to make do with a mid-table amble to retirement.