Being an optimist requires a level of irrational thinking. Pessimists at heart are rational people, who chose to see the world as it is.
Optimists are irrational people. They refuse to use conventional thinking and accept the world like it is but chose to take crazy large risks in order to create what could be.
Take Elon Musk, for him to be able to think that he can build an electric car company, he needed to be irrational, and challenge conventional thinking. And it is these people who chose to not follow the fold and forge their own path who create the future.