Why Don't Single-celled Organisms Need A Mass Transport System?

Single Celled Organisms do not need a transport system simply because they are 1 Cell and because of that they never developed one, oxygen,water(by osmosis) and other needed substances simply diffuse into the cell and unneeded substances diffuse out.
I didn't use scientific terminology to give a simple answer.

Single-celled organism do not need a mass transport system because, they have vessels which transport oxygen separately from the blood.

Because they do not have a long distance to travel and transport anything