The games engines are not written in RuneScript, but instead Java. RuneScript was developed by Jagex to allow their other staff that do not know java to make and add content to some of their big games. The game engine then reads the RuneScript files and executes them accordingly, this structure is how most games and most big programs are written, so that someone can change, for example, a file called 'monster1.xyz' and the main program reads that new file.
Quote Mod John A:
We write in a scripting language called RuneScript, which our Game Engine team has created specifically for making RuneScape content. The Java-based game engine then reads this RuneScript code and makes it all work. RuneScript is constantly being changed and expanded as we add new features to it to allow it to do new things.
This was taken from New Dwarf Quest: Coding and Writing