they're called essential because humans cannot produce them internally, so we have to consume them (though you could in principle make the same assessment for other animal species, but that's less relevant, unless you're, I don't know, raising cows?)

plants don't eat, but produce organic molecules from raw ingredients (or almost raw, in case of nitrogen), and can produce all amino acids - but in different quantities, so maybe the (parts of) plants you eat don't have all the necessary amino acids.