Honest question: what is it about the standardization that fundamentally makes standardized tests easier to prepare for vs. non-standardized tests? It's not like either sort of test re-uses questions year-to-year, or that non-standardized tests have mystery subject matter.

The standardization is that everyone gets the same questions, or a mix from the same pool of questions. That aspect doesn't seem to make preparation any easier or harder, unless you're talking about the standardization making elaborate cheating schemes scale better, but that's not generally considered prep.