[chmod775]

Because in English you say "three dot two", whereas in German it is "Drei-Komma-Zwei".

It just reflects the spoken language. And having the unused symbol then be the thousand separator is natural.

[usui]

Interesting, I did not know this, but a little bit doubtful. Wouldn't it be the other way around? Explicit spoken language coming from being written that way.

[chmod775]

Maybe at some point originally, but now you can't change it. Spoken language resists attempts to shape it by committee, and written language has to begrudgingly follow its lead.