Every delivery company I've worked with will only acquire a signature if the sender has requested the service (unless UPS or FedEx to a business address, where it's included). Signature confirmation is expensive (at least $1 extra to postage), and so most senders will just ignore it, especially for low-cost purchases. Yes, they definitely should spring for it with something like a PS3, but some sellers just don't care that much.
In situations where a signature is not required, it's often up to the delivery person him/herself (or policy) to decide whether to leave the parcel on a doorstep or return it to the delivery office. Since it's often more work for all involved to take it back to the office, most carriers will just leave it if they don't have a very good reason not to (like creepers watching for lost parcels and stuff). And 99% of the time, that works just fine. And the other 1% is not significant enough to anybody but the buyer to really worry about. Sucks to be that buyer though, cause insurance is a PAIN.