When the detector goes off, the real work starts: removing the device by hand or, preferably, exploding it in place.
That metal detector is a lot more sophisticated than you think. For example, way back in WW2 the Germans started using wooden box mines where the only metal was the detonator made of brass or copper. The metal detector is useless if the mine isn't in the ground -- for instance, a Claymore-type mine by the side of the road or in some bushes or a tree or other places wouldn't be detected.
Then consider car bombs and truck bombs, suicide bombers, and the rest. The detectors you describe aren't exactly feasible for them.
There are two things that really do work: bomb-sniffing dogs and human awareness. When you see something that doesn't belong there, or in some way arouses your suspicions...you call the EOD fellas or blow it yourself. Better to blow up ten pieces of junk than miss one live one.
Then, of course, there are the command detonated ones: cell phones are used these days, for instance, and have themselves been turned into bombs.