Because WoW isn't worried about making the game fair for everyone, not every spec can clear every raid but every spec tends to have a use even if it's outside of PvE.
And perhaps more importantly none of the specs in WoW have 3-minute 20-button rotations so they're easier to balance and design with customisation in mind.
You mean exactly what XIV does with crafted gear being able to be pentamelded?
