I did some editing to try to make that part easier to understand:
What do people do about that? They multiply by weighting factors that get results they think are reasonable. But that isn’t actually a way of making decisions. How do they know what’s reasonable? They must be using their intuition, common sense or something else other than weighted factor summing. So the weighted factor summing method doesn’t work as a self-contained solution to decision making. It relies on pre-existing opinions reached some other way. People often make the math (or non-numerical estimate) come out to fit what they already think (without realizing they’re doing it). For example, college rankings often start with the pre-existing idea that Harvard is good and then give high weightings to whatever factors Harvard is good at so that Harvard-like schools come out on top, which seems like a reasonable conclusion to people who already believed that Harvard is one of the best schools.
Any method involving arbitrary choices (like what unit conversions or weights to make up) runs into a major problem: You have no good way to make an arbitrary choice unless you have pre-existing knowledge of what a good answer is.