Dogs have been eating plants for as long as they have been domesticated. It’s a trait that’s been passed down to them from their ancestors. Dogs can be trained not to eat plants around the house, though.
Another reason why your pups might be eating plants is that they’re not full! They might still be hungry and eating plants help them fill up their bellies.
One other reason is that they’re bored! A bored pooch is a nibbly pooch. So, best to train or redirect that energy toward a toy!