I know, I know; both of them have a life obsession. Black pays it a lot, White makes it a lot. And lifelink can show up anywhere at all. But still, the point is more one of 'Sorin is a creepy bastard and both of them resist magic', combined with the spell itself being a quick thing. Saber's magic immune unless she wants to be affected, Archer can ignore things cast so hastily and still reduces the effects of bigger stuff. Even if it WASN'T so quick, spreading it amongst so many people would dilute the effect. Also, they really don't need physical buffs with their weaponry. XD