Why doesn't a boost converter help overcome USB cable resistance?
You have discovered the negative resistance feature that occurs with switch mode regulators.

This is a very simplified explanation of what is going on. For all intents and purposes and assuming a fixed load, you can consider the regulator to be a constant-power device. If the input voltage increases, the input current decreases. The opposite occurs when the input voltage decreases - the input current increases.

Now consider what happens when the load current increases. The input current has to increase to handle increased load current. But: the resistance of the USB cable means that the input voltage decreases. Recall that I mentioned that the input current has to Increase when the input voltage drops.

This is a vicious cycle and results with the voltage input to the regulator dropping to a very low value. 

There are a couple of ways to fix the problem. 

1)  Add a pair of heavy wires in parallel with the power conductors of the USB cable. 

2) Use a boost converter at the source end of the USB cable, followed by a buck converter at the load end. In other words, feed the power input of the USB cable with a voltage that is high enough to overcome the wire resistance.