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.
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?
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."