Why does the OSI reference model have seven layers, not six or eight?

Because it was develop in that way. Nowadays, layer 2 is sometimes divided into 2 "sub-layers", and the MPLS protocol is considered to be between the layers 2 and 3, and is therefore sometimes called "layer 2.5", but the traditional 7 layers are usually maintained as the "official" layers. We might say this is for historical reasons.