The Island was not purgatory. They were alive on the Island.
"They’re all dead too?"
“Everyone dies, Jack. Some before you, some long after you.”
Some before you: Boone, Sayid, Sun, Jin, Locke, Shannon, Charlie, Juliet, Desmond, etc.
Some after (off Island): Kate, Sawyer, Claire, Miles, etc.
Some after (on Island presumably): Hurley and Ben
"The most important part of your life was the time that you spent with these people."
So, obviously Jack (and everyone else) were alive on the Island.
"This is the place that you all made together so that you could find one another…That’s why all of you are here. Nobody does it alone, Jack. You needed all of them and they needed you."
As a result of Jacob and the Island, the castaways’ fates were intertwined and dependent on each others’. They spent the most meaningful part of their lives together. Why not enter the next life in the same fashion???
I’m sorry, but the people who think the Island was purgatory are the reason I can’t have nice things.