Actually, 20,000 feet is a common enough certification standard and is likely to be quite arbitrary. If there is no requirement to extend the test parameters beyond that, why go to the expense?
Like the common 8 or 10,000 feet bird impact speed limit with windshield heat inoperative. But AFMs may be written that this limit is 10,000 feet AMSL rather than AGL, so what happens when you are over mountains where eagles fly? Common sense prevails, one would hope......