there is pressure in the feedline but no nearly as much once u get into the center cartiridge of the turbo. think of it as a faucet and sink kind of. theres pressure in the faucet but once it sprays out into the much larger sink it looses most of its pressure and velocity. then it goes down the drain "aka return line" and then into a sewer i guess"aka oil pan".

the inside of the turbo isnt not meant for such high pressures and if the oil sits there to long it will fry from the extreme heat the turbine creates. so if u rely on pressure to push the oil out its gonna eventually blow past the seals in the turbo or just cook inside the cartridge and smoke like a cheeba. thats also the reason why the return line is also a lot larger than the feed...so the oil can get in and out quickly..

tried to put it into simple words so its easier to understand