Because Primary & Foreign Key information is not enforced.
"Primary and foreign keys are informational only and are not enforced" from:
https://docs.databricks.com/en/tables/constraints.html#declare-primary-key-and-foreign-key-relationships
https://www.databricks.com/discover/pages/optimize-data-workloads-guide#delta-data
Delta data skipping automatically collects the stats (min, max, etc.) for the first 32 columns for each underlying Parquet file when you write data into a Delta table. Databricks takes advantage of this information (minimum and maximum values) at query time to skip unnecessary files in order to speed up the queries.
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