Probably because folks like Mike Okuda, who "came up" through fandom thought all ships had unique emblems. Until the Justman memo came to light, I think 99.9% of fandom thought that.
I concur but IIRC this concept was suggested by the various different insignias featured in TAS.
Back in the 1970's and prior to TMP, TAS had a quasi-official canon status and the amount of TAS materials Bjo Trimble covered in her Star Trek Concordance is a testament to that, IMHO.
And unless I'm mistaken Geoffrey Mandel's popular Officers Flight Manual also featured individual (conjectural) starship insignias.
Bob