Because the Bantu languages (most prominently: Swahili) and Japanese have similar sound systems. Finnish is also oddly similar-sounding, or Hawaiian. None of them are actually related. It's because the syllable is restricted in the number of possible forms, in a similar way. (And they all have approximately five vowels. And a pitch accent.) In Hawaiian, nothing but consonant + vowel syllables are possible. Swahili an…
Anyway, I think there's another factor: the common alphabet we transliterate these languages to is quite limited. I suspect the similarities become less obvious if you use something like the IPA, which has better universal correspondence between sounds and letters (i.e. doesn't reduce every sound to the same ~26 symbols).