That assumes a level of planning an forethought that some if my players lack.an 11th level wizard... with a what for AC? 11?!? I mean at least give them mage armor... you know the spell they can cast, last all day and not have to concentrate on for the cost of the lowest spell slot... I mean most damaging 1st level spells are out done by cantrips and if it saves 1 shield spell it is worth it.
It's also fairly typical for PCs that don't wear heavy armor and shields.
One of my assumptions is that the guidelines are for casual players that are not particularly optimized.