The term "toast" originally referred to a specific process of cooking bread, which involved holding a slice of bread near a heat source until it became brown and crisp. This process dates back centuries when people used to toast bread over an open flame or with the help of heated metal grates or tongs.
Over time, the word "toast" came to be associated specifically with bread that had been subjected to this browning process. The verb "to toast" also came to mean the act of making bread brown and crispy through exposure to heat.
As language and culinary traditions evolved, the term "toast" started to refer not just to the process but also to the end product - the browned and crispy bread itself. When we refer to "toast" today, we are usually talking about the finished product that has gone through the toasting process.
So, to answer your question, "toast" is not called "toasted bread" because the term "toast" evolved to encompass both the process of toasting and the resulting product, which is the browned and crispy bread we enjoy. It's a linguistic and culinary convention that has been passed down through generations.