How come the Hebrew words for 6 and 7 are so similar to their Latin counterparts, while the other digits aren’t even close?

There has been speculation that Indo-European borrowed its words for ‘six’ and ‘seven’ from Semitic, or that they reflect a common ancestral (Nostratic) element. Nostratic is not a mainstream theory, and there has also been significant scepticism about borrowing, especially if the Proto–Indo-European for ‘six’ is closer to *weḱs than *sweḱs.

I’ll note that PIE borrowing numbers from Semitic is not as implausible as it might seem. There is widespread suspicion that PIE borrowed some words from the contemporary prestige languages of the time, Sumerian (isolate: https://www.ling.helsinki.fi/~as…) and Akkadian (https://books.google.com.au/book… ); the words for ‘silver’ and ‘bull’ are the most commonly invoked of the latter. And in a time of early numeracy, borrowing the words for ‘six’ and ‘seven’ is plausible.

See also

• Why the Hebrew Numbers Six and Seven Sound Almost English.
• The etymology of six
• The etymology of seven