Why doesn't the same amount of oil exerts always the same pressure at the bottom of a container of any shape?

This is a good question. The short (but less satisfying) answer is: "because the hydrostatic pressure of oil at a certain depth depends on the height of the oil level, measured from that point." So, you can have a tiny amount of oil but arranged in a column that makes it very tall/high and get high pressure, and have same amount of oil spread so that the height is negligible to produce a negligible amount of pressure. The exact formula for pressure is P=r*h*g    (r stands for the liquid density, h for height, and g for gravitational acceleration).

The long answer, but much more satisfying, goes through the derivation of this formula. I recommend searching for a good video explaining hydrostatic pressure. (I can't post links).

good looking response