They have protein but they're mostly composed of complex carbs like starch, fiber, and cellulose. Their actual protein content varies but in general it's much lower than legumes

If you're vegetarian or vegan and don't eat legumes, then you're probably not getting enough protein.

I try to teach people about protein and bioavailability all the time!

https://en.wikipedia.org/wiki/Liebig%27s_law_of_the_minimum

https://en.wikipedia.org/wiki/Digestible_Indispensable_Amino_Acid_Score

And a very handy tool - https://www.diaas-calculator.com/

While many plants have some amino acids, they don't have enough of the right combinations to be biologically useful (liebig's barrel). It's dangerous to assume all plants are protein, because that is a measure of crude protein (nitrogen estimate) and not actually amino acid ratios. Make sure whatever plant your eating actually has a biologically meaningful diaas score.