One explanation I've heard for not using the OS user's locale for the language is the "foreign webcafé" use case. Imagine you go to a web café in a foreign country, whose written language you don't understand, to check a news website. The OS is probably configured in kiosk mode and for the language of the foreign country, so you can't change the OS user's locale/language setting. If the browser and news website just use the OS user locale language setting, then you won't be able to view the news website in English, short of resorting to Google Translate.
Contrariwise, there's also the case of the bilingual tourist, whose mobile device is set to use their native language for comfort, but they might prefer to view websites related to their trip in the local language even if the sites are multilingual, because the translated versions might be incomplete or the translations might be of poor quality, etc.
I'm guessing that's what the browser vendors have concluded, for whatever reason, though I can't recall any browser that's attempted to expose any language selector button or menu item outside of their Preferences windows (and of course average users never touch the Preferences).