As with the design of operating systems, it makes sense to distinguish between policy and mechanism. Science and rationality may or may not be a sufficient basis for creating policies. And here by "policy" I mean things like "people should be equal before the law", "healthcare should be a right", "what's good for Wall St. is good for America", "all citizens should be armed to the teeth", "Mars colonization should be our highest priority". That is, policies are goal statements, and reasonable people can certainly disagree about what our goals should be as a society. Mechanisms are the means we use to achieve our goals, that is, the means by which policies are implemented. So a policy might be: "wealth inequality should be bounded", and mechanisms to achieve it might include "progressive income tax", "subsidies for the poor", or "universal basic income". Given a policy, science and rationality are certainly applicable to designing and evaluating the efficacy of mechanisms to achieve the policy.
Our biggest problem is that most of our political discourse is consumed with debating what we call policies, but which are usually mechanisms to achieve some policy. The policy is seldom explicitly stated and almost never debated, while the participants in a typical political debate take it for granted that everyone accepts whatever implicit policy their proposed mechanism seeks to achieve. But even worse, we implement mechanisms without ever tying them to an explicit policy goal, which makes it difficult to determine whether a given mechanism is working. Politically you get a situation where anyone who questions a mechanism is assumed to be disagreeing with the unstated policy behind the mechanism. The result is that bad mechanisms become entrenched, and are no longer subject to rational or scientific examination. And that just sucks.
If I could interject one question into every political debate, it would be: what are you trying to achieve? And if I could have a second question it would be: how will you know if you've achieved it?