Exactly, essential words that we needed thousands of years ago as well as today sound similar. Because they are leftovers from when it was a single language. Modern words that were just invented tend to sound similar as well. E.g. "computer" in different languages. The vast difference is in words that came up in between split of Indo-European tribes and modern times.

The written numerals are relatively recent, but the words for numbers are much older and still very similar across languages.

Wasn't Roman numerals in use, before the Indo-Arabic came to Europe? How come they are similar? They look worlds apart to me..

https://en.wikipedia.org/wiki/Roman_numerals

These are the words the Romans used:

unus duo tres quattor quinque six septem octo novem decem ...

Yes you are right.. indeed sounds similar..

The numerals are different but the words are the same. 5 and V are both pronounced “five”, just represented differently.

Ah my bad...