Because 90% of the time the bad guy knows it takes 20 seconds, and has a stopwatch by the phone, and hangs up at like 19 seconds, just to toy with the good guy.
Which is why, back in 2007, the NSA infiltrated stopwatch manufacturers and altered their timing mechanisms so they run slow - when a stopwatch says 19 seconds, in truth it's been a bit over 20 seconds. So now when the bad guy thinks he's outsmarted the beautiful police detective, she's had time to set up that GUI in Visual Basic and knows exactly where he is.
Don't believe me? Just look at the number of world records that have fallen during the last few Olympic Games.