Post Made Community Wiki by pyon
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Several reasons:

  • The person using the browser may not speak the language that the browser uses. (Say, someone who is French on an English machine...)
  • The location/language information may not be accurate (for any number of reasons: VPN, proxy, etc.)
  • You could call it laziness on the part of the developer(s), EXCEPT that it is always best not to ASSUME, and that I might very well be interested in a country / language other than the one my browser is in.