The Mercator projection models the earth as a cylinder. I think the association here is just that people are most familiar with the coordinate grid as represent

The Mercator projection models the earth as a cylinder. I think the association here is just that people are most familiar with the coordinate grid as represented on the Mercator projection, which creates the illusion that lines of longitude are parallel. This illusion is all the stronger because it makes spherical coordinates look like the more familiar cartesian plane, which would tend to be most people's first intuition before remembering that the planet is roughly spherical. The situation is even a bit worse in marine or air navigation because the nautical mile is (or more accurately was) defined by a minute of latitude, which is nearly consistent across the globe and creates the temptation to measure along longitude the same way. In reality nautical and aviation charts have to use a specific conical projection to keep this naive way of measuring distances approximately accurate.