Why wasn't michael and Walt in the finale of LOST?

I was wondering that too...Now, this is not the thought of my own, but it's a good answer to your question. It helped me understand.
"1st you have to understand that Nawawala is the story of Jack's life and the connections he's made in his journey. The alt reality is not another dimension or reality at all. The "alt reality" was unrealized death. All the back stories on other character's were to ipakita us the events that brought all of them to Jack. Michael died on the island and his spirit was trapped there (in the real world) because he killed Ana Lucia and Libby. That's why we didn't see him in the alt reality. Walt had left the island as a young boy and most likely had made much stronger connections with other people other than Jack. That's why we didn't see him in the alt reality. All the people in the alt reality were always dead... they just didn't know it. That's what Desmond was doing, helping them to let go of their "physical presence" mindset and see that they were dead. Once they did that, they could all pass on to a higher place. And they all needed to pass on together."
Post sa pamamagitan ng Billy

Darlton have been very clear about this... Walt was not in the final scene for purely logistical reasons. He was too old when they filmed the church scene. They would have loved for him to be there but it would not have looked right. Remember, Aaron who we've seen as a 3 taon old in the series, was seen as a baby again in the church. It was a collection of the people that Jack needed and that needed Jack, time stamped the way they looked on the island prior to Jack's death. As Aaron was a baby on the island when he left, it's why we see him as a baby in the church.

Same for Walt, except that the actor was as tall as the rest of the cast sa pamamagitan ng the time that scene was done in 2010. It would not have looked right and too inconsistent with the other people there.

It's in large part why they created the 12 minuto prologue 'The New Man In Charge' where we see Walt in 2010 taken back to the island sa pamamagitan ng Hurley and Ben.

As for Michael, he's not in the flash sideways at all because he is on the island in 'purgatory' for the bad things he has done - so there's no way he'd be in the final scene.

Because both characters were unpopular with fans - especially Michael. And both Cuse and Lindelof wanted to kowtow to the fans' desire for Michael to suffer punishment for his actions . . . forever.

ben gave them a bangka so they left

Michael was dead and Walt was out of the picture back in civilization with his grandma.