Because that worked out so well in the movie

found on eBaum's World

What the Oakland Police want is a way to kill people without putting their officers in even the slightest bit of danger. They want the big bucks for their dangerous job, without the danger part. Unfortunately this really doesn't eliminate the danger, it merely transforms it. 

You see, there's only 2 ways this armed robot could work - either it's autonomous (sort of like the Terminator in the movie was) and can direct itself and decide who to shoot, or it's remote controlled by a human operator (which would probably be more likely). Either of these options would be dangerous. The autonomous one could decide wrong and shoot it's handlers or innocent bystanders. Meanwhile the remote controlled one could be hacked and used against it's handlers or innocent bystanders.