I worked at Google in 2000, and I found this annoying too, so I asked about it. The official reason was that a lot of browsers defaulted to en-US, so Google effectively treats an Accept-Language of en-US as a no-op, and then it falls through to location-based. (I don't agree with this rationale, and said as much, but there you have it.)
Note that other values for Accept-Language are not ignored. I haven't tried it myself, but I've heard some people say that setting their language to en-GB or en-CA works well.
You can also use &hl=en-US, which always takes precedence.
One sad thing is localization for Canada is kind of broken for a lot of platforms it seems? For a lot of people that I help having their locale has Canada seems to usually base it off of United Kingdom stuff including setting paper size as A4(!)
Which… I wish we used but unfortunately we don’t - we use the US paper system. this causes problems with stricter printers and printing and also just manufacturing stuff like PDFs
Note that other values for Accept-Language are not ignored. I haven't tried it myself, but I've heard some people say that setting their language to en-GB or en-CA works well.
You can also use &hl=en-US, which always takes precedence.