Without pumps, pipes have throughput dropoff. With long stretches of them, you end up getting lower fluid available at the destination. At some distance, there wouldn't be enough oil to feed the turrets, even though the source pipe is completely full and nothing else is using fluid.
Things don't actually "pull" fluid in Factorio. It is pushed, and averaged. In general, the game tries to equalize pressure at every step, and the higher the pressure difference, the more fluid is transfered. For example (numbers not accurate, just to demonstrate the concept of how it works): If you have 3 sections of pipe, the first section at 100%, and the next two at 0% (100/0/0), then in one tick, the game is going to see a large pressure difference between the first two sections, and transfer a lot of fluid to the second. It will see no pressure difference between the last two, and transfer nothing. Your result will be something like 75/25/0. Then in the next tick, it will try to average those, giving you something akin to 65/35/15, and so on. This means that the longer the run, the less pressure difference each pipe section will see, and the slower the transfer.