Because it's a role reserved for the primary DPS classes (warriors,rogues,hunters) which have no other options. That is our JOB if you will in the arena it creates a good synergy in group composition where you want a healer you want a healing de-buff and you want a utility class.
Grab a healer
Grab a healing debuff
Grab a caster/utility hybrid
This is what helps keep representation among the classses balanced.
Hunters Rogues and warriors should only be able to fill the healing debuff role, You shouldn't be able to grab one of us to be your utility.
Healers fullfil the healing role. You don't grab a healer to CC for your team.
Hybrid classes have healing and dps trees and their class can fulfill 2 roles in a group depending on their spec which is why there are more class/spec options for the 3rd hybrid slot in a group.
No offense but your analysis is extremely narrow minded. Many healers can play offensively with cc and utility. Ms classes bring utility through things like lock down, interrupts, snares, and cc of their own (blind, traps, aoe fear (has to be willed or tremored). Who are you to say a 3s team can't consist of 2 casters and a high-cc healing class, (druid-lock-mage) or a 3 dps team with 2 casters and a rogue/hunter. You can't expect everyone to basically play the same "balanced" set up.