See World of Hyatt Globalist late checkout - bogus and inconsistent.

The only legitimate reason I think can of is that the property is classified as a 'resort', even if it's not obviously so. In the rare circumstance that I've been denied a valid late checkout request, I escalate to the manager on duty and either reach some compromise (EG move to a regular room for half a day), or request their reasoning in writing so I can escalate to Hyatt.