Yes mercator distorts size at high latitudes. But it preserves angles which is more important for many (most?) mapping applications. Literally no one is using a mercator projection to determine size at the continental scale. We use mercator for navigation and for local maps. It keeps everything at right angles. Roads that run N-S look like vertical lines. If something is to the lower-left, that's south-west. These characteristics make it ideal for most maps.

The UN has only one mapping application - to equitably portray the world's country boundaries. In the larger space of potential maps, this is pretty niche. As a result, they need a niche projection.