Just added spoil tags to your post just incase
That is exactly why a lot of people hated the finale. They didn't understand it. The Island wasn't purgatory, but the 'flash sideways' (like the shows flash backs and flash forwards' were a different universe (the purgatory universe) that ran parallel to the main story. The Island existed. Everything that happened, happened.
The “flash sideways” universe introduced in season 6 was a sort of stop-over point between life and afterlife (referred to here as the “purgatory universe”).
Each person in this “purgatory universe” created a reality for themselves based on their lingering issues in life – that which they could not “let go” of. For Jack it was Daddy issues; Kate, the guilt of murder; Sawyer, the quest to find “Sawyer” and be a better man; Sayid, the unrequited love of Nadia; Charlie, looking for something “real” in his hollow life of fame, etc…
Everyone was still attached to their Earthly concerns (we’re getting very Buddhist here, bear with me) – but when they made contact with those people they’d met on the Island, they remembered the journey and growth they had experienced because of the Island, and could finally understand the connections and “purpose” brought into their damaged lives by being there. With that greater understanding of themselves, they were each ready to “leave” or “move on” to the next phase of existence – i.e., the true afterlife.