As we get older, our perception of time speeds up. An immortal would easily lose track of time after just two human lifetimes, causing an immortal to suffer from dementia-like symptoms where they expect one date but find themselves habitually late. And since time doesn’t mean the same thing as us to an immortal, they would eventually become disconnected from the world around them and be unable to reintegrate. They wouldn’t be able to maintain friendships, relationships, mortgages, payments, etc. They would be surrounded by people but forever alone.