Unfortunately, there are a number of unscroupulous individuals looking for 'easy/quick money' and they simply follow a delivery truck and wait for a delivery to a semi-obscure residence. On the other side of the coin, there are people who willingly commit fraud and claim their package was not delivered and demand a replacement. I'm sure that UPS and Fedex have dealt with all kinds.
But on the other side of the coin, there are less than completely honest delivery drivers. The UPS driver I assisted on most days of the 3+ weeks I was a driver helper groaned every time I brought the box back to the truck and left a tag when nobody answered the doorbell. A driver I spent one day with insisted after I brought back the first box that all other signature required packages be delivered and I simply scribble unintelligibly some fake signature. At Fedex, the driver I trained with for a week would take the package back and leave a tag the first day, but scribble something and leave it and a tag the second day. Of note is that Fedex (and I think UPS, too) have removed the option on the door tag for the person to sign the tag and leave it on the door the next day. A thief could easily see the tag the first day, take it, and put it back the 2nd day with a fake signature and the driver would leave the package, oblivious to the fact the 'real' person didn't sign the tag, and the package would be gone shortly thereafter.
Bottom line, although 'signature required' is a good method to assure successful delivery, there are sometimes failures. Safest bet...make sure someone is home to receive the package...or, have it delivered to your workplace.