Because they are made of wrought iron and the dog probably cannot eat them.
Here Scout the Hungry (black lab) and his accomplice Max the Digger (tan and black) are contemplating them:
He might pull it off. After all, Scout has eaten everything else, including:
This kid’s shoes:
That kid’s shoes:
The house and the outlet cover.
The back of the bar-be-que (hence the chicken wire):
Sprinkler system. ($90 repair.)
The box for the new wrought iron table.
A lot of rocks.
Some fence pickets.