What is tartan if nothing other than cloth with a woven pattern ? What I have read about it is that during the Jacobite rebellions, chiefs liked to look as unworldly as possible with the patterns they wore - and the usually wore multiple different patterns too. These same chiefs were often European educated and perhaps had to work on their street cred with their warrior clansmen.

Weaving a complex pattern is time consuming and expensive, so why would they take such time before the invention of proper looms or without a chief with the money to pay for it ?

Clans had emblems and heraldic arms anyway, so they would have sufficed for identification purposes.