From a personal POV abilities should remain excluded, primarily because of Mold Breaker and its variations.
This means that ability granted immunties are not reliable because they can be negated. Also Levitate is not the only ability that grants an immunity; Wonder Guard, Water Absorb, Volt Absorb, Sap Sipper, Flash Fire et al. grant immunities as well, and many Pokemon who have these abilities have other abilities (so your argument that adding them would be easy doesn't hold water).
Of course, type granted immunites can be negated, but only through moves/ items that are seldom used.