Why aren't popular compatibility tests scientifically validated?

Because they're popular, not because they work. MBTI was created in 1942 without scientific basis; it's intuitive so people use it. Love Languages has no research backing; it's memorable so it sells. These tests are useful for self-reflection but not for prediction. Only Big Five and Gottman assessments have serious research support.

What makes a compatibility test scientifically valid?

Predictive accuracy (it actually predicts relationship outcomes in new couples), consistency (it measures the same thing across people), and transparency (it's studied independently, not just by the creator). Most popular tests fail these criteria.

What should I use instead of popular tests?

Honest conversation and observed behavior. Do you share values? Can you communicate about conflict? Are you both willing to grow? Does this person show up for you consistently? These matter more than any test score. If you want assessment: Big Five for personality, Gottman exercises for communication, or couples therapy for comprehensive evaluation.