If you don't believe me, construct a parser to correctly decipher the meaning of an English sentence, or to reject it if the sentence is not valid.
It's hard. Very, very hard. English is a mess. It's one of the hardest languages to learn for a reason: it's huge, and it's inconsistent, even compared to other natural languages.
We have a whole host of ways to write negation: "not hopeful", "atypical", "unrelated", "disproportional", "counterintutive", "antiestablishment". That's not an exhaustive list. "Up" opposes "down", but "to burn up" is not the opposite of "to burn down".
The language is ambiguous, even at it's most basic levels: In the sentence: "He finished his bow, and walked off briskly", the actor could be a bowyer or an actor, but we can't know without more contextual information. Languages that resolve ambiguity due to context are harder to understand: English does it a *lot*. And despite training, we can't really manage to learn it all.
Everyone in every English-speaking nation that I can think of is legally required to attend a *minimum* 11 years of schooling, where they learn spoken and written English, and formal rules of grammar and composition. Despite this, few native speakers understand even the most common English language idioms.
For example, I don't know how many times I've read people who should know better write: "to tow the line" (to tug on a rope!) rather than "to toe the line" (to step carefully along a pre-set path) as an idiom for obedience to authority. They should know better after 11 years: but they don't.
That's only true if you only learn one way of saying things, or a small sub-set of the vocabulary. Then you can't understand what someone else is saying, if s/he chooses to use a different phrase.
Name ten people who know every single word in the Oxford English Dictionary, every meaning in every context. I don't know any; and I know a lot of teacher and English majors. I doubt you know anyone who does, either. Everyone speaks a subset of English; and that's a problem. Try talking to someone straight from Jamaica: you won't understand him. You won't understand someone straight from old-world Scotland: as one of my friends was quite rudely told, "Yae dinnae ken hae tae tawk raeght!"
English is a mess. Worse, yet, literary English is confounded by a whole host of metaphor, much of which only makes sense only if you have a grounding in classical mythology. If you disagree, well, perchance we might stridently take discourse in the temple of Mars, good sir! Engarde! :-P