I think the answer to this question can be found in the Introduction Chapter of the Documentation. It states that:
One major difference to other engines is that the 2D Render Engine is entirely based on plain Java AWT Graphics. If you've learned or starting to learn Java this will instantly give you great results and highly optimized rendering performance with what you already know. We think that this is a great and simple way to start making video games without having to care about a lot of vector math or "OpenGL shenanigans".
The LITIengine is tailored towards people with a Java background that want to get into making video games. It also targets beginner-level programmers and if you learned Java at school, college or any beginner course, you've probably done something with AWT already.
For advanced/professional level game-making, OpenGL or Vulkan is probably the way to go but for the target audience of this engine we decided to keep the rendering pipeline based on AWT. Actually, in the past we've had this discussion a few times but ultimately decided against abstracting the render pipeline for the mentioned reasons. Maybe some day in the future, we'll revisit that topic but for now, the LITIengine is an AWT based game engine.