Why is it that a red blood cell is shaped like a disk?

For function. The purpose of the RBC is to transport oxygen via the hemoglobin/Fe molecule. In order to optimize the amount of hemoglobin present the cell is devoid of a nucleus and cytoplasmic components. This allows the maximum number of oxygen molecules to be present in each RBC. The cell has the other job of being able to circulate via the microvessels to deliver the O2. Its disc shape provides entry.