Ben told Jack that if Jack would do something particular for him, Ben would personally take him off the island.
Now we find out that the particular thing is to perform surgery on Ben to remove a tumor on his spine.
Here’s the question: if Ben needs surgery and has the ability to get off the island (like he promised Jack), why doesn’t he just leave and get surgery off the island? Why risk staying on an isolated island to try and convince a survivor surgeon to save him?
This is one reason I trust Juliet, not Ben.