I wrote a long answer about biology and evolution, and then I lost power. Too lazy to write it again, but long story short:

We don’t and can’t really know exactly why. Teeth are weird and grow in many shapes and sizes. It is also often not as defined as in that diagram, some horses don’t have it and the ones that have it may not have it on all the supposed teeth, and it is not considered a reliable way to age a horse anymore.