Canon focusing system

Excuse my ignornance, I'm merely an eager student.

I have seen a video of the 5D MK III new focusing system with its 61 points. I have a 7D which has 19. I was expecting the 61 points to take up more area of the view finder. I'm wondering why it doesnt have focusing points covering the entire view finder area.

Is that simply impossible or just unnecessary?

It's a matter of getting sufficient light to the AF sensor. It's a limitation to the current DSLR design.

That said, the new 61-point AF that the 1DX and the 5D3 uses have the widest spread of all Canon FF DSLR line-up. It's wider than the old 45-point. The 7D and APS-C cameras can get away with what appears to be wider (in the viewfinder) because of the crop factor and the resulting narrower FoV.