" * I have to wonder if the same effective earth ground can be obtained by encasing a #4 AWG conductor or a 20' piece of 1/2" rebar in the concrete slab of a basement. Why does the UFER ground need to be in the footing?"

I believe the intent is to connect to the existing rebar at the point at which there is the highest occurance of moisture in the surrounding soil. I believe the pressure of the structure resting on the footer would enhance the connection of the concrete-encased electrode to the earth.

The more rebar that is tied together, the better and more redundant the connection to earth would be. Therefore, in my opinion, the use of the rebar would be a superior connection to earth as opposed to the 20 feet of #4 method.

"What about burying a 20' piece of rebar 12" in the dirt, then pouring concrete over it?"

That would not be a CCE according to 250.52(A)(3), as it is to be encased by 2" of concrete. If you pour over rebar laying on the ground, then it's not encased: the underside is not in contact with the concrete.

"Would that not provide an effective ground?" Whether it did or not, it would not be considered a true grounding electrode by the NEC, in my opinion.

In my opinion, the rebar is to connect the Grounding Electrode Conductor to the concrete, which in turn in porous, which in turn absorbs water, which all taken together makes a killer connection to the earth.