What am I missing?

[Lalaithion]

Yes, it does matter. At the equator, the "box" bounded by a small equal angles of latitude and longitude is approximately a euclidean square. As you move into higher latitudes, it narrows and becomes more like a trapezoid, until you get a very narrow triangle at the poles.

A map in a projection suitable to a restricted extent is going to try to minimize distortion within that extent so those shapes will be approximately preserved: At mid latitudes, a lat-long box with equal angles would be approximately a rectangle with an aspect ratio determined by the latitude. In a normal cylindrical projection it will be projected as precisely a rectangle.

In tangent normal equidistant cylindrical, all such boxes are squares, but this means that they, and all other shapes, are being stretched out east-west. Which is a rather noticeable form of distortion. So my point stands, to get a square lat-long graticule, you can either be covering a small area near the equator in a projection that minimizes distortion within your extent. Or you can be in Plate Carree, in which case you can be anywhere, but would have significant and ugly distortion unless you are covering a small area near the equator.

The equator is geographically significant, yes, but it mainly just is another invisible line on our map. You could center a equirectangular map anywhere in the world, effectively treating any great circle (or really, any circle at all!) as the equator and any two points as poles- here we see the cassini projection of our world:

Which is basically the equirectangular projection with the prime meridian as the equator (it is called a cassini projection, just like how a plate careé is just an equirectangular projection centered on the equator).

(I agree with you that it is anachronisctic, but we can make exceptions in order to provide our readers/players/others clearer maps.)