Earlier quoted context omitted.
Because that prevents all of your users from selecting the language they want. It's a terrible idea with no upside and not-high-but-still-not-no downside.
It doesn't, because that's an optional negotiation. Try Apple.com in a different country/locale than yours, you'll see how it behaves.
The page defaults to the locale that you request in the URL. https://www.apple.com/ shows up in English, regardless of your country;† https://www.apple.com/bg/ shows up in Bulgarian. Switching your preferred location simply takes you to the page for that location. (Dyson does the same thing.) Some locations support more than one language; there's https://www.apple.com/lae/ for Latin America (English) and https://www.apple.com/la/ for Latin America (Spanish). If you're on the page for a location like this, a language selector (with language names) displays next to the location selector. In the case of Latin America, only two languages are supported, and the language selector automatically displays "Español" if you're on the English site and "English" if you're on the Spanish site, which makes sense but won't generalize.
Apple's selector is inconspicuous because it refuses to display flags, which I would guess is due to much higher political exposure than Dyson. So it's lower-quality in two ways, but fundamentally the same approach. The user asks for a language, and the site honors that.
Given that I presented Dyson as an example of doing language selection correctly, I'm confused about what you wanted me to see on apple.com. They're trying to do the right thing, but less effectively.
† I tested this by accessing the site(s) from Mongolia, Vietnam, and Morocco using ExpressVPN.