i am on 44 hours awake so i will say this simple because i may fall asleep while typing this or i am so tired i won't make much sense

if everyone had the ability to use any skill they wanted, every troll would use provoke and tanking would be impossible, everything would use manthra making healing so easy thatt hard things don't be come a challenge, they did it to add challenge to your team as well as, some aim of adapting with your class. and also to stop people from min/maxing the class and being extreme elitist.