That just seems odd to me to have a wire for earth... carried above the earth. I can understand having earth wires in houses made of concrete and wood since they have to get to the earth through those materials, but what makes a common earth ground better than just lots of many grounding points? All that wire has got to get expensive.

See here: https://en.wikipedia.org/wiki/Single-wire_earth_return

The earth is a pretty terrible conductor.

The point of an earth ground is to dissipate the accumulation of static built up. Ground is not connected to the power supply so there is nothing to return to the power company. The earth is the best place for it.

Depends on where you are in the country. Here's some 3 wire I found by dropping a random pin: https://goo.gl/maps/MkoiprJ1gWM2

I've always remembered it being 3 pin around where I grew up.

That's a higher voltage power line (in the low kV range) with just the three phases. They're relying on the load being approximately distributed between the three in the downstream distribution network to obviate the need for any return wire. If you move forward one step on that road you will see the next pole has a lower set of wires which will be at 230/240 volts. On these it looks like there are four wires, they're probably doing without the earth wire in this rural area and just tying to earth periodically.