The Romans figured this stuff out over 2000 years ago, it's not that hard. Fluid likes to flow downhill with gravity, if the tank (inlet) is higher than the outlet it all works fantastically, if not, you have to wait until complicated 18th century steam pumping to make it all work, and that even just involved pumping it back to a height it could flow downhill again. So you pick it up from a single sump where gravity deposits it and pump it upwards to a point it flows down again. All extra complications and weight you don't need in a simple light aircraft. Any extra holes in the system between point A and B add complications where fluid will either leak out of or air leaks in.