Why don't USPS, UPS, FEDEX etc. ring your doorbell anymore when you get a package?

Answer 1

My guess is that it depends on the person delivering your mail or package. I myself ring the bell and if I don't hear a doorbell, then I immediately knock. I've had packages left by UPS, etc and they don't always knock; just leave the package.
Like I said to begin with; it depends on the person doing the job on how curtious they want to be.

Answer 2

Check the wiring in your doorbell. I always ring. If your Letter Carrier doesn't call the post office and report the person. I don't know about those other dudes but there are USPS rules for how to handle that kind of delivery.