It only lasts for 5 min because everything in FFXIVs battle system is based off the system then had in alpha when each and every skill gave you a small amount of SP.

cast protect gain 50 SP
cast shell gain 50 SP
cast cure gain 150 SP
cast blizzard gain 25 sp.
mobs dies you gain a total of 275 SP.

SE hasn't updated the mechanics of the battle system other than adding a stamina bar to "hasten" the feel of battle.

Having to cast shell and protect every 5 min doesn't make the game harder, and having them last 30 min won't make the game easier. Having to actually use your other skills like crowd control or enfeebling, or hate control with your nukes make the game more challenging.