I'm guessing this is the case because "I didn't see the email" is plausible. I think all my jobs have asked for a one sentence resignation email to HR just so everyone's on the same page.
It has nothing to do with the plausibility of seeing the email or not. Somebody could say “yeah I read that email. It was stupid and I wasn’t going to respond to it. I would still like to work here though, just not on double hours.”