Because GTB limits entirely anything the mage class can nearly do, or Ninjas, or um Priests, or um...well alot of things if you make it a 1vs1 fight it is not right to have mvp cards almost to be fair. MvP cards are dealable in a slightly larger pvp surrounding where there would be counterbalances to it.
Sad thing about GTB is there are not enough effective counters to it to even give the poor spellcaster types a chance =/