Why are airspaces depicted in AGL?
You're confusing the airspace height with airspace altitude.

The definition of Class C (in the US) is 


  3-2-4 (a) Generally, that airspace from the surface to 4,000 feet
  above the airport elevation (charted in MSL)


If you look at chart for the Class C for Colorado Springs (elevation 6,187), the surface area of the airspace goes up to 10,200 ft MSL, approximately 4,000ft above the airport elevation of 6187.