They do this to spare themselves the cost (if any) of a phone call, probably from overseas. Why? Well, there are far too many curious people who can't resist calling the number back. That puts the number into the network for spammers to call and call and call and call and ... you see where it's going.
DON'T CALL BACK when you get such a one-ring call, or for that matter, a call from an unknown number on your caller ID. You have caller ID for a reason. Use it!