You know, before I took 30 seconds to come up with a handful of other inconsistent pronunciations and begrudgingly concede that it wasn't any weirder than a lot of other inconsistencies I'd already absorbed without really realising it.
> NEVER learn a new word without learning what is its sound.
Because on probably a monthly basis my partner will make fun of me for pronouncing something wrong. The phrase is "I learned it from reading" --but it's not a good enough excuse anymore in our wide web'd world.
Oh, and I've spoken American English since birth, and have been variously told my language and communication skills are very strong.
The reason why they're pronounced differently is that they have different language origins. Arkansas is from the plural form of the French name of a Native American tribe, so you don't pronounce the '-s' while Kansas comes from the English spelling of a similar(?) tribe, where the '-s' is pronounced. English is notorious for just borrowing words from other languages wholesale but it seems to work most of the time.
Honestly though, mispronouncing words is a rite of passage for ordinary native English speakers too, especially ones who acquired a large chunk of their vocabulary from reading.
It is pronounced goat-em.
Other nearby places include:
Loughborough, pronounced luff-bruh
Leicester, pronounced less-ter
Derby, pronounced Dar-bee
It wasn’t until Beyoncé became a thing that some people started getting it right. And even then, I’ve still had to prompt them — “you know, like Beyoncé?”
This example also happened to me (native British English speaker), mistakes will be made if you don't hear how a word is pronounced.
But occasionally, it happens that I still get it wrong. For example, just a few weeks back, I've heard 'Arkansas' pronounced in the News - totally different than I would have pronounced it. I know how to pronounce 'Kansas', and I was always under the impression that 'Arkansas' is the same with an 'Ar-' prefixed, which is apparently not the case.
Since it hasn't been mentioned yet in this thread: https://youglish.com/ is absolutely brilliant to figure out how something is pronounced, saved me quite a few times already of mispronouncing things.