Because it takes 10-15 seconds to hand over a govt-issued photo ID, have it reviewed, handed back and pass on to the screening. Conversely with the database check, it means showing up with backup documents or having faxed in utility bills, magazine subscruption labels and items which correlate to the commercial databases. That takes somewhere in the 5-10 minute range, access to a terminal and the time of a trained employee.

Somewhere in the system, the pax name needs to be matched to an assurance that the pax really is who s/he says s/he is. There are all kinds of reasons why people might assume false identities. Some of them of questionable morals, but not unlawful and others unlawful and dangerous.