A proper and intuitive UI is one of the hardest and most time consuming things to do. From a design perspective, it takes a lot of effort, trial&error and user feedback to find out what works and what doesn't work, and to create something that's coherent and intuitive, rather than a patchwork of individual and differently behaving tools. From an engineering perspective, it's a very time consuming task. The work of doing a proper UI may be even bigger than the work on doing the actual image processing and other software core engine tasks.
There are plenty of drawing applications that have great UIs. The easiest solution for GIMP would be to copy one of them (arguably Photoshop, because it's closest in terms of expectancy / user-base).
