Why each time of kernel compilation must be done completely?

Hi, This problem has puzzled me for a long time. A normal C make file could parse the reliance and just regenerate the modified source file. But each time when I modified the kernel and recompile it, it does a complete compilation, no matter whether make mrproper is used or not. It's quite time consuming and inefficient. So why kernel make file couldn't do what a normal C make file could do? Or maybe it could but I got something wrong? If a complete compilation is inevitable, how to make it faster? I only know the -jX option. Best regards Michael