Ask Apple, i don't know the details of the implementation or design decision. Maybe it doesn't lose it, but Apple still requires the passcode after a reset, for whatever their reasons.

I agree with you that it is possible to do what you suggest (long secure password, assuming you don't lose it), but very few people do it or will ever do it.

https://www.quora.com/Why-does-touch-ID-on-the-iPhone-5S-ask-you-to-enter-a-password-after-restart-and-not-just-allow-fingerprint-scanning-alone

We're veering off topic in any case, I think, unless Monero has similar issues.