First off: I’m not really well-versed in terms of UX design, so these next guesses are as good as anyone’s:

- Six boxes communicate “== six digits, != password” (try to imagine the least tech-savvy user)

- Some people might not be able to quickly hold six digits in their head at once (especially seems to be the case with older people, from my experience), so seeing “I’ve already got two/three/four” visually is potentially an efficiency boost. This also correlates (again, just from my anecdotal experience) with the population that doesn’t have a password manager handling 2FA for them automatically…

- This is probably down to preference, but IMO it also feels faster somehow than a single input field with six digits, when it works properly (with the caveat that it feels much worse/slower if it doesn’t work)

- Now that everyone does it, you’re kinda forced to adopt it as well for recognizability.

But it’s an interesting question – we can’t ever imagine entering postal ZIP codes like that and yet that’s an area where it’d actually make more sense since there’s usually some notion of “more significant” digits at the start vs. end, so you could perhaps do some cool tricks with regard to pre-filtering state, city etc. names. Whereas OTP codes are (AFAIK) essentially fully random digits with no meaningful distinction.