You can make fully rational decisions in a small, closed, predictable universe.
But most situations aren't that. There are too many variables, unknowns, and unpredictability. The source and purpose of life itself are on shaky grounds, causing any rationality on top of it to be baseless outside a defined scope. Evolution through randomness, not logic, is the nature of our reality.
Most big and hard decisions are based on some rationality but largely by instinct or circumstances. When a decision succeeds, there's post hoc rationalization and further success. When it fails, as most do, they disappear and are forgotten. This gives an illusion of a rational and deterministic universe.
The best you can do is reason and decide on the high-order bits and then go with your gut for the rest. Make bets proportional to your risk tolerance and resources, make multiple bets, bias toward action, and course-correct along the way.