Why is Arabic twice harder than Hebrew, as hard as Chinese? Hebrew and Arabic are closely related (the alphabet (well, an abjad) is almost the same, grammar is almost the same).
For example, a Moroccan Arabic user would have trouble conversing with a Tunisian Arabic user, even though both countries are in North Africa. On the other hand, a Saudi Arabic user likely wouldn't even be able to tell Moroccan Arabic from Tunisian Arabic[1], let alone understand a Moroccan or Tunisian Arabic speaker!
The further east/west you go, the more differences arise (pronunciation, vocabulary, and grammar), and the harder it becomes for Arabic users to understand one another. But it can get even more hairy: in some cases, Arabic users from the same country can face trouble understanding one another in day-to-day speech.
Given that the goal of an FSO is to understand as much of a language as possible (I assume?), learning Arabic becomes a much more difficult task. If the goal was just to learn MSA (Modern Standard Arabic), which is the formal and written form of Arabic, then I think it would fall under the same category as Hebrew. But MSA can only take you so far and won't really help you integrate with the local population.
[1] Middle Eastern TV channels (e.g., Al Jazeera) typically include on-screen captions in MSA if the speaker is using an "unintelligible" dialect such as Moroccan Arabic.
I imagine different regional accents in Russian could be the same for non-native speakers. What a native speaker does not even notice could be completely unintelligible for somebody with just few hundred hours of studying the language.
Things like Arabic verbs having 3 letters. Everything is limited to 3 vowels.
Some letters are t, th, d, dz, dh. There's a light and heavy h. a, a', ak, aq are different letters. Length of vowels also determines the meaning of a word.
Gendered and plural are handled together.
And a lot of it is very poetic. Arabic is one of those languages that is incredibly well designed for poetry. English is the least poetic of the languages I know; good for getting to the point. It's hard to explain, but there are a lot of things, like double meanings of words and phrases, connection between action and subject, flow of one concept into another.
tldr: it's really unintuitive to the native English speaker
No it's not. A non-fluent Hebrew speaker would not even be able to tell the difference between the modern and biblical. It's the same words, the same word order, the same letters, pronunciation (modern abbreviates sounds a bit, but the abbreviation is more slang than official - if you ask someone to speak slowly then the pronunciation is the same in both).
You'd have to be pretty fluent in Hebrew to even be able to identify where there are differences (mainly in knowing which words are considered archaic, and some sentence structure that is not common in modern Hebrew). But to actually know those details requires a deep knowledge of the language - a beginner speaker would never be able to tell the difference.
For all intents and purposes it's all one language. It's not like English where middle English is basically unintelligible.
There are tons of people who only ever learned the biblical Hebrew, and they can speak to people who only know modern Hebrew - they sound a bit funny, but that's about it. A beginner speaker would never notice.
> so it did not have time to accumulate weird exceptions
There's been plenty of time, but it hardly happened for a different reasons: The language is basically defined by the Torah and the Mishnah (and other bodies of work). The way it's written there is definitive. So it's difficult to create exceptions when you have so much written literature that does it in a particular way.
A notable exception is uncommon and technical words, which have definitely changed over time - but a beginner is never going to encounter those.
As a non-fluent Hebrew speaker who took ~10 years of Hebrew classes as a kid, this seems totally untrue.
Biblical Hebrew is intensely weird. It doesn't use the same word order (VSO vs. modern SVO). There are differences in grammatical system that, if you don't know about them, make it look like past and future tense are reversed (https://en.wikipedia.org/wiki/Waw-consecutive). The vocabulary is more limited but the words get used in a wider range of meanings.
Random example, Genesis 2:15:
וַיִּקַּח יְהוָה אֱלֹהִים, אֶת-הָאָדָם; וַיַּנִּחֵהוּ בְגַן-עֵדֶן, לְעָבְדָהּ וּלְשָׁמְרָהּ.
A fluent speaker would know, I'm not arguing that. But a beginner? They'll hardly get it right in the first place - they won't be able to tell which dialect it is.
My point is not that there's no difference, it's that the difference is minor in the context of language change.
In modern Hebrew it would be something like "Hashem lakach ha'adam v'sam oto b'gan Eden..."
The grammar required for "vayikach" to make sense is missing in modern Hebrew, which I remember being confusing even in elementary school. This isn't a niche case since a huge fraction of the Bible uses verbs like "vayomer".
Also, the end of the sentence, while comprehensible, still feels very archaic.
I don't think biblical Hebrew qualifies as an entirely separate language but I do think even beginners almost immediately sense the differences. It's like comparing modern English with Shakespearean English.
Lastly, a note on pronunciation: the language used to span a huge phonemic range (roughly the same as Arabic), but we ended up dropping like a third of the sounds. So, sure, modern pronunciation of biblical Hebrew sounds like modern Hebrew. But that's kinda tautological.
This is not true at all