As your voltmeter shows, all of the main wires are hot. No electricty flows because there is no connection to ground. It has potential to flow, but isn't flowing. It needs a path to ground (and ground to battery or alt ground)
It's like the water in your house. All of the supply pipes have pressure in them. All of the faucets are shut, no one is doing laundry, so no water is flowing.
With no water running, pressure is pretty much the same any place we measure it.
What happens if a garden sprinkler is turned on?
->Water flows through hose to the sprinkler. Pressure in the hose goes from zero to same as the house. Water comes out the sprinkler and falls to the ground. House pressure stays about the same.
Electric current is like the water flowing. If water pressure in the house is 90 psi, its not going to drop much on the way to the sprinkler. Even washing potatoes in the kitchen sink, the pressure will not drop noticebly between the entrance and the faucets valves unless there is some sort of resistance such as caused by badly undersized pipes or heavy mineral build up.
The battery is like a storage tank. So if the house was using water from a storage tank, then as the water level drops, pressure will also drop. But as long as all the faucets are shut, all of the pipes will be at same pressure as the storage tank's outlet.
The alternator is like a well pump. It can supply water at higher pressure than the tank, but only as long as the amount it can flow isn't exceeded.
Hope that makes sense.
