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I think the answer is simply: because they feel like it.
US CBP (Customs and Border Protection) officers have an unusual amount of leeway and there is little oversight, so the experience tends to be very inconsistent and unpredictable. The "why" is often a moot question since the answer is simply "because that's what the officer wants to do".
I've seen officers that behave professional and courteous, I've seen officers that were outright abusive. I've seen people getting the Nth degree interrogation, I have seen people being waved through without looking at ANYTHING AT ALL.
I've seen an officer in secondary inspection ripping a page out of a non US passport. That's clearly illegal, but there is no recourse and no consequences for the officer.