Casting focus is not meant to be a guaranteed way for casters to avoid damage.
At 5% a player has a 1 in 20 chance of continuing to cast after being hit.
The only guaranteed way to avoid interruption is to use protection.
Instead of providing casters with a reward to allow them to avoid interruption, casting focus is meant to occasionally forgive a caster for mis-timing their spells.
Additionally players with journey man or above skill level in inscription receive a base casting bonus boost which also allows them to exceed the casting focus cap of 12%.