Your job as a software developer is, roughly, to deliver maximum business value with minimum cost, and that includes writing minimum code to accomplish the task. That is what the senior developers are talking about when they're saying you've written too much code.
It does sound to me like they've got a well designed system if you can add new features simply by making a small modification and some configuration to the existing code. As long as it's also well tested and well documented (that may be lacking...), you've got the ideal codebase: one in which you can add large amounts of business value with only incremental changes.
You mention that this is "great for the business but not much fun for me". I think this is a sign of inexperience. Part of what makes something fun is how difficult it is. If something is too easy it's boring; if something is too hard it's frustrating. In the middle we find things fun. The reason writing lots of greenfield code is fun for you is because it's easy. Understanding and modifying existing code is harder, and that's currently too hard so you find it frustrating. You need to push past this stage to improve as a developer. As you get better, it will become easier to work in existing code and will stop being so frustrating.