Because plants are not animals, and we are? "Nearly all foods contain all twenty amino acids in some quantity... Proportions vary, however, and most plant foods are deficient in one or more of the essential amino acids."[1]
[1]: https://en.wikipedia.org/wiki/Complete_protein
Deficient means there are lower levels of certain amino acids, but they're not missing entirely. It wouldn't make any biological sense for plants to be missing amino acids. If you read the actual cited source, you will find this:
> Usual dietary combinations of plant proteins are complete; specific plant proteins may be low in specific amino acids
> There is no evidence that amino acid imbalances per se are important; possible imbalances can be created by inappropriate amino acid supplementation, but this is not a practical problem.
