Why aren't snails and slugs often seen eating plants?

They don't get eaten as much because sea slugs are toxic and are dangerous to eat while snails, on the other hand, are not.
The sea slugs eat the stinging anemones. The sea slugs even eat the stinger! However, instead of digesting the stinger, it goes into their cerata, which are finger- like parts on their back that help with digestion, breathing, and defense. When an organism chews on the sea slug, they use the stingers as weapons.