I'll be using Arabic as my counter. As far as we know, Arabic at its core has had the same standards of writing as it has had 1500 years ago. The Qur'an, a 1500 year old text, can be read and understood by pretty much any fluent Arab.
Yet English, in comparison, has changed quite a bit over the last millennia. Take this, for example--a sample of Old English from around 900 CE:
Drihten me raet, ne byth me nanes godes wan.
And he me geset on swythe good feohland.
And fedde me be waetera stathum
Meanwhile, reading the Qur'an or other similar texts by Arabic speakers is no harder than an English speaker reading texts from the 20th century. Is there any explanation as to why that is?
The Qur'an, a 1500 year old text, can be read and understood by pretty much any fluent Arab.
Where have you read that? As I understand, it's very hard for modern Arabs to simply pick up and read like any text without any training in or knowledge of the old Arabic it's written in. Perhaps not completely illegible but still very difficult.
I've been thinking over your question for the past couple of days, and thinking someone else would come along and answer it, but they haven't, and I think it deserves better than you've got so far.
So, (1) I recommend you actually repost it on a linguistics sub (such as /r/linguistics or the smaller r/asklinguistics/ - just make sure to read their posting rules).
(2) Here's my answer...
Your question actually has far more to say about Arabic than it does about English. English has experienced considerable linguistic evolution/change over the last 1500 years, but not in any way a suprising degree of change. All languages changes over time, though not always in the same way and definitely not at the same rate. The factors involved can be complex, and I would ad that English in recent years has become so fluid because (a) it is a language with so many speakers, and (b) a lot of 2nd language speakers, that small 'change effects' get amplified.
What about Arabic? I presume you were taught Modern Standard Arabic, and probably write in MSA, though depending on your context there's a good chance you don't speak how you write.
In any case, there are all sorts of varieties of 'Arabic', and all of them show linguistic evolution from Classical Arabic (the Arabic of the Qu'ran). Even MSA, which self-consciously models itself on the standards of CA, does not adhere to CA in all aspects.
Here's where and why Arabic is a fairly unique cases - the Qu'ran and Islam function as very powerful 'conservative forces' in regards to language. The Qu'ran doesn't get translated, it gets 'interpreted'. If you are serious about Islam, you learn Arabic to read the Qu'ran. And not just any Arabic, you learn the Arabic of the Qu'ran. So for 1500 years there has been a very, very powerful socio-religious force that instills a particular 'standard of language' in almost all Arabic literary education. That applies to 1st language speakers in schools, and 2nd language speakers. It's why MSA remains consistent across countries too. It's a linguistically 'ideal' language.
And, it explains why diglossia is so common for Arabic speakers - one language (= version of Arabic) for the Qu'ran, high level literature, government, politics; another language (= version of Arabic) for everyday life. Both get called 'Arabic', but linguistically speaking local dialects of Arabic can deviate very far from MSA.
Arabic, then, is the outlier. While there are other languages with high degrees of linguistic conservatism, there are none that I can think of that have it for the same reasons. Certainly, for English there was never a single "linguistic ideal" embodied in an unchanging text that required all religious adherents to learn a single-version of English. That linguistic force doesn't hold English together.
Like I said, I do suggest cross-posting. I'm very happy to talk about linguistic change/factors in English/Arabic/other languages some more.
You might also try /r/linguistics.
Do you have examples from other languages? I was under the impression that Arabic was the odd one out here, not English.
Basically it has to do with the homogenity of a society.
English in 900 CE wasn't English, it was Anglo-Saxon. When the Normans invaded in 1066, they brought with them French, and French and Anglo-Saxon mixed to eventually become English. English has a long history of absorbing words and phrases from other languages, and given how far the British empire stretched, this is not surprising.
On the other hand, the Vikings settled in Iceland around 800 CE, and remained an isolated, homogenous society. How isolated? Well, today Icelanders still use the patronymic, so you have to look up names by first name, and Icelanders can read the Nordic sagas in the original while modern Norweigians and Danes can't.
Another example - for many years Hebrew was considered a 'dead' language, since the Jewish diaspora would acclimate to whatever language was being spoken and pretty much only used Hebrew for liturgical purposes. However, the creation of the State of Israel caused a revival of the language - to the point that you can meet modern Israelis today who only speak Hebrew - and while biblical and modern Hebrew are not the same, they're still fairly close.