The the idea of simplicity in software engineering is one of those hand wavy stuff that the conventional wisdom is full of.
The simplicity (or the complexity) depends on what you want to do (requirements). If the requirements themselves are complex then expecting the code to be simple just doesn't make sense. Also piece of code would look simple to a person who has been working on it for long time and has the background knowledge vs a person who is new to the system.
Another thing to remember is that software systems are built incrementally i.e for each new requirement the developer have to figure out "how to implement this new requirement in this given code base with minimal changes" and this eventually will lead to complexity (unless you want to do big refactor on each new requirement).
The simplicity (or the complexity) depends on what you want to do (requirements). If the requirements themselves are complex then expecting the code to be simple just doesn't make sense. Also piece of code would look simple to a person who has been working on it for long time and has the background knowledge vs a person who is new to the system.
Another thing to remember is that software systems are built incrementally i.e for each new requirement the developer have to figure out "how to implement this new requirement in this given code base with minimal changes" and this eventually will lead to complexity (unless you want to do big refactor on each new requirement).