The main reason why it is not realistic to measure the distance between two points on a map as a straight line is because a map is a flat representation of the Earth. However, the Earth is not as flat as the map. Instead, the topography is complex, with slopes, hills, etc. These geographical features can change the speed at which we travel, the path we need to take to get to a different point, and the distance between such two points.
The map is a flat schematic of some actual distance, on a map, the distance form point A to point B may well be 10 units say, however, in actuality, if you go and walk from point A to point B, you'd notice that, first off is not a straight line most of the time, and also it may have a slanted or sloped section, and the map is not including that slope or hill, whilst you do have to walk it.
so, adding all the twists and curves along the way and any bumps or slopes, is really not exactly 10 units from A to B.
