Why Doesn't the City Contract for Itself?

One of the pethora of questions we ask ourself is: WHY DOESN'T THE CITY SET UP IT'S OWN DATABASE OF TERP JOBS? The City of New York spends millions of dollars each year for ASL interpreters. (And should be spending more, since many of the Deaf are not receiving adequate service.) While the city -- and businesses -- tend not to complain about the lawyer fees they have to cover, they love to complain about what they spend on interpreter services. So why doesn't the city just set up a database of interpreting jobs and let interpreters fill the jobs charging their own fee. i.e. 50-70 dollars an hour for certs, as opposed to 70-90 an hour the agencies charge? This would save hundreds of thousands of dollars a year. So what's stopping them? Ideas?