It is easy to make a decision when you know the outcome. Most things that people want to change in the past are single moments that don't require any follow through. Would you really have changed if your older you came up to you in the past and said, man brush your teeth and eat healthy? Unlikely. But if your later self gave you winning lottery numbers or tells you to buy bit coin. The at something you would do, but you can't make these changes today because you don't know the outcome.