Most birds cannot easily peck through the shells.
So, the bird has to be big enough to grab it with its beak, then it flies up and drops it onto the driveway. Then flies down and eats it.

There are many bugs that would eat a snail, like beetles, and a shell is almost absolute proof against those.

And, we have really come to the point in development where armor is almost irrelevant.
Armored vehicles need lots of protection to reach the point in a war where they can be used.