The decision does get made. It just gets made in bits: a chat thread on a Tuesday, an email with six people on it, a quick chat after stand-up and a review the new PM wasn't in. Three months later a developer asks why the checkout button sits where it does (or a stakeholder asks, or a new hire), and nobody can point to the answer.
So the designer defends the same work again. The PM digs through old messages to piece the story together. And the team spends an hour re-making a decision it already made, because the "why" never got written down next to the design.