Three main reasons...
- Top level containers aren't double buffered, which cause flickering when the frame is repainted, yes, you can implement you're own double buffering, but...
- Painting inside a frame does not take into consideration the frames borders, meaning that it's possible to paint under them. Frame borders are also platform/look and feel specific, meaning that there sizes change
- In the case of Swing windows, there are components that already exist on the window, meaning that they paint over (or be painted over), in most cases, both (because of the optimised painting engine in Swing), making it difficult to produce a reasonable result
For point #2, frame decorations are painted WITHIN the "window" bounds, not outside them
Take a look at...
- How to get the EXACT middle of a screen, even when re-sized
- Java AWT drawString() does not display on window
- How can I set in the midst?
for more details
Make the time to read through Painting in AWT and Swing and Performing Custom Painting for more details
