Because you (and most people) misunderstand how targeting works in D&D.

When you cast a spell it asks "Is this target X?" where X is whatever the spell is able to effect, say intelligent undead.

With willing only spells it asks "Is this target willing (unconscious or choosing to forgo their save)?" and if the answer is yes then the spell works

Will all other spells it asks, for example, "Is this target a creature?" If it get's a yes it then asks "Is this target immune to this spell, say [mind affecting]?" if it receives a no then it asks "Is this target willing?" if the answer is no then it gets a save.

A creature must be unwilling for a save to be rolled.