People asking for a magic casting tank aren't taking something else into consideration. Outside of being interrupted by being hit (And thus needing a passive that prevents this), there's also the very important factor of movement. Tanks generally don't have the luxury of being able to continuously stand still in order to cast things. They're moving often to avoid enemy attacks. From bosses, to massive pulls that are pretty standard when doing dungeons. The more enemies you have on you, the more often you're going to have to move. Or how about in PvP, how will this tank perform it's job in PvP if it cannot be standing all the time to cast abilities, given that game mode is a very mobile activity. The end result would be a tank that nobody would want for that content or has significant problems that other tanks handle easily. Making such a problematic concept is directly contradicting how SE has chosen to design it's jobs so that everything is viable. Even very minor weaknesses in jobs results in a damaged reputation and stigma towards that job by the rest of the player base that is hard to change.

The solution to this problem would naturally be either: A) This magic casting class has no real important abilities that rely on cast times, which begs what would be the point of even having those casting abilities. B) They have another trait/passive that enables them to cast while moving, which again then it's not particularly a caster, just a regular job that has a wait time on their abilities to go off. Or C) Their spells are instant cast and thus the job is absolutely no different then things like we have with DRK where they have magical themed abilities that do AoE etc.

Once again, the bigger picture needs to be looked at, things need to be considered. The same goes with melee healers which simply will not work in this game, especially when there are fight mechanics that require the healers to move away from the rest of the group (Ifrit EX, Alexander Gordian 4) and other mechanics where additional players in melee range will only severely complicate things resulting in a healer nobody would want in their group over a WHM/SCH/AST.