For comparison, another monster type that has CON scores but is immune to crits is the Ooze. Oozes are essentially undifferentiated masses of cells, or perhaps even gigantic single cells, to use modern terminology. Because they're undifferentiated, they're immune to critical hits, and have no front or back to speak of. But since they're cells, they are very clearly alive, and have metabolism and health like more complex creatures do. Therefore, they have CON scores. Elementals are basically the same thing, just using a different body medium.