The Sahara Desert.

Most of the straight line borders run through that territory, and when delineating mostly uninhabited territory between settlements, it's often easier just to draw a straight line. Cyrenaica and Nubia (or Libya and Sudan) have long histories of being seen as separate culturally from Egypt (if it times politically dominated by strong Egyptian states) but the southwest corner was a matter of drawing lines on a map south from the coast and west from the Nile - Egypt's Matrouh and New Valley governates, which cover those straight line borders in the south and west, have collectively under one percent of the country's population.