I just realised that all (standalone) Japanese numbers from 1-10 are borrowed from Chinese (maybe except 4 and 7 if they're read as よん and なな instead of し and しち). Now, I understand why a language would want to borrow words for really large numbers (like European languages do with Latin words - billion/trillion/quadrillion/quintillion/etc.), but why would a language borrow words for such simple numbers as 1-10? Why aren't 一、二、三、四、五、六、七、八、九、十 pronounced in a native Japanese way as ひと、ふた、み、よ(ん)、いつ、む、なな、ここの、とお instead?
Besides prestige reasons, there is also the fact that the Old Japanese numeral systems can be seen as inconvenient, especially for higher numbers. Disadvantages compared to the Chinese system are:
names with different lengths. While Chinese numerals are single-syllable (in Chinese) and have 2 morae (besides 2,4,5 with only one mora) in Japanese, there is a bit more variation in Japanese numbers, with 1-3 morae. This is kind of a weak argument, but it could be that a single number word carried too few information to be useful to act as standalone number.
The system is far less regular. 20 and 50 had own names, 30, 40, 60 were formed with "-so". Depending on the counter, some numbers had different forms, as can still be seen in the difference between say ７日（なのか）and ７つ（ななつ）.
But what is worse is the way how you combine places: I'm no Old Japanese expert, but at least the examples on https://crd.ndl.go.jp/reference/modules/d3ndlcrdentry/index.php?page=ref_view&id=1000101493 seem to indicate that the counter had to be repeated in words like 「54種」「イ(50)+クサ(種)+アマリ(and)+ヨ(4)+クサ(種)」.
That leads us to another possible reason, that is differences between script and reading. If you see 「54個」and know Chinese, it should be much easier to pronounce it as 「ごじゅうよんこ」than as the Old Japanese counterpart.
Rather than reforming one's own system, it is always easier to borrow another system, and since Chinese influence was big, it is possible Japanese started to use the Chinese numbers as convenience where big numbers where needed.
The reason is similar to the reason why English has borrowed (French) words for beef, pork, mutton even though there are Germanic words for cows, swine and sheep. There is a tendency to borrow words from languages up the social scale, and at the time, Chinese was the upscale language, just as French was the upscale language in England, or Arabic was the upscale language in East Africa (hence analogous number borrowing in Bantu). There is a contemporary trend in modern Bantu languages to take numerals from Swahili (ergo a second round of Arabic influence). 一 can be pronounced hito, it's just a complex sociolinguistic question (with grammatical overtones) to determine when people are more likely to use the historical Japanese form.
They are pronounced that way, before the importing of the Chinese characters, and even to this day: ひとつ、ふたつ、みっつ、よっつ、いつつ、むっつ、ななつ、やっつ、ここのつ、とお.
There even is the shortened way of counting: ひ、ふ、み、よ、い、む、な、や、こ、と.
It's just that the Chinese way of counting has also become popular, but it doesn't mean they don't use the Japanese reading as well.
It's not only Chinese numerals that were imported to Japanese, but also Chinese counter suffixes (these are akin to units and are specific to the category of object being counted). Now how could something this complicated and foreign to the language take hold? By means of the education system. Numbers beyond the smallest ones are not learned as part of acquiring the language, but instead are taught in school or, more generally, the system of education. Education in counting is of specific use in administration and business, formalized education was specifically aimed at a future in administration. The administrative language of Japan was Chinese until at least the Edo period. Hence, all educated use of numbers was Chinese or Chinese adjacent. Without a formalization of Japanese counting and math, and the writing thereof, there simply was no way for Chinese numerals to not become standard for anything but the simplest uses of numbers or numerals.
Same as in Thai. The numbers 3-10 are derived similarly to Cantonese, probably because Chinese traders influenced numbers across the region.