It's true because the evidence indicates it is true. At least to the overwhelming majority of people who have studied it.
Evidence is evidence. It doesn’t “say” anything. This is the fallacy called “reification”.
Evidence must be interpreted, which it is by the observer. But the observer inherently assumes a “lens” through which to interpret the evidence. Honest observers understand this.
Self-ignorant observers/interpreters just accept their assumptions as true without question.