How come many receivers don't use a ground?

Almost all two pronged appliances use the grounding plug method where one pin is larger than the other so it can only be plugged in one way. This is to ensure that the body of the unit is not closer to the hot side than to the earth ground side. Other than that, all components are ground referenced to each other through the use of shielded cables. Equipment is much less susceptible to ground noise than it used to be as power is filtered and reformed by almost all the components in your stereo individually. Phono players would still have to be grounded to the amp or preamp they are connected to, but it is just so they have the same ground reference to reduce hum and rumble, not to ground the turntable itself. The reason you need to ground a turntable is because the needle is not electrically connected to the turntable. The only thing that is powered in the turntable is the motor to turn the record, not any of the audio path. In all your other stereo components (CD, DVD,etc.) the audio connections have a ground reference on the shield portion of the connector, phono uses a differential, or balanced signal, with no ground, so the ground pin is to provide audio reference ground between the devices.