Because the CPU used is only rated for use at a maximum of 600Mhz. That's basically it. It's the safest option because they are guaranteed that every single CPU will run at that clock speed for at least a certain amount of time. I, for example, can't run at 900Mhz; so having a default speed of that would be great fun for someone like me :P

Be thankful you have such an open device that lets you overclock so freely :P