They know. They also know that almost no regular users know what it is, what it's for, or how to configure it. The search fails where the accept-language was wrong far dominate the numbers over the search fails where the accept-language was ignored.
> The search fails where the accept-language was wrong far dominate the numbers over the search fails where the accept-language was ignored.
Wouldn't this imply that "regular" users are running their OS and/or browser in a language they do not want to use for search? This seems unlikely to me, or is there some systematic reason that results in this being the case?
Browsers tend to default to an Accept-Language of en-US, and most users do not know how to change it.
As a result, Google learned early on (and numbers continue to suggest it is the case that) an Accept-Language of specifically en-US really means "The user has not chosen a language; fall back to a heuristic."