Florins are the de facto standard in MTW/M2TW because they were the de facto standard among many nations in medieval europe.

They are not made up, and they are not being turned into something they weren't. Most of the centers of merchant exchange and more importantly banking were in Italy. Therefore most rulers had bank accounts in Italy and the accounts were denominated in Florins. This is also why so many nations ended up calling one or another of their own currencies florins.