Because if you do then thieves will ask to take a test drive and when your dumb rube self says "OK, here are the keys," they will drive away and never come back. This is apparently happening frequently enough that the NYPD issued a memo about it. So that's too bad. Not that cars are getting stolen! That is part of the great shadow economy that we all unwittingly feed off of. No, what is too bad is that there are people letting Craigslist danger-strangers drive off in their cars with the promise of returning them. Are they all Rip van Winkles who woke up and didn't understand how the internet does? Or are they old people whose grandkids set up the ad and then walked away, callously assuming that gramps could figure the rest out for themselves? A little of column A, a little of column B, most likely.