I'm not sure exactly what you mean by "graphics from outdated times", but I assume you're talking about low resolution monochrome. The reason is rather straightforward: they still perform a useful function at a price people will pay. A high resolution color display exacts a price in processor power, memory, battery life, and cost. For less than $150, there is still a market for a pocket sized device that can handle some pretty heavy number cruching and give a meaningful output, without being loaded down with other resource drains. Flashier is not always better. In this case it's efficiency that wins.