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Because it is added once and never checked. A template change breaks a property, the rich result quietly disappears, and nobody notices for months because nothing errors.
Structured data validation checks the schema on each page for syntax, required properties and agreement with the visible content.
Every crawl validates the markup and compares it to the rendered page. Schema that no longer matches what the page says is flagged, since that is a manual action risk rather than a warning.
A page's FAQ schema still lists three questions that were edited out of the page. That is flagged as a mismatch, not merely as valid markup.
It will not guarantee a rich result. Eligibility is necessary and not sufficient, and Growth reports eligibility rather than implying the outcome.
Syntax, required and recommended properties, eligibility for rich results, and agreement between markup and visible content.
Those the engines document, including FAQ, article, product, breadcrumb and organisation.
Yes, and reports issues by template, which is usually where they originate.
No. It validates what is there.
Fourteen days, every module, no card. Or half an hour with someone who will run it on your own records and tell you where it does not help.