Because security isn't perfect and attackers need only one mistake. There were some early computers built with security in mind, yet their cost and performance

Because security isn't perfect and attackers need only one mistake. There were some early computers built with security in mind, yet their cost and performance were impractical for most uses. There is also the problem of everyone having different needs, so permission profiles must vary widely. OS builders cannot assume anything about the limits of their use. Finally users can be the weak link. They may not understand the risks of what the screen is asking them when prompted.