1 week of supplies is more about flexibility. In survival situations once things get bad they snowball. People aren’t going do drop dead on day 8, but think of it as having a week to prepare for a known disaster before you need to worry about the basic necessities. 6 months is hypothetically better but far more expensive and it would only make an actual difference in a tiny number of situations.

Further it’s fairly trivial to cycle through a week of supplies and keep it fresh. Looking at the shelf life of what you regularly consume, find stuff that lasts a while, get a little extra, and then keep restocking as you normally would after that point. Just remember first in first out.

PS: It’s honestly surprising how long some stuff lasts, honey in an unopened glass container is going to be good long after you’ve died of old age.

At scale, no response to a major emergency is ever fast. Everyone having 7 days would give the authorities at least some time.

There's a lot of governmental preparedness in place with a response time of say 2-10 days (depending on specifics). The idea is to carry over until communal response kicks up and starts working; not to hole up in the woods by yourself for months, that's an Americanism. Nordics are not like the US; there's no separate paid military, everyone is part of defense, and all crisis planning is about defense in depth, which includes protecting civilian infrastructure.

To give an idea of the amount of preplanning and resources to be deployed in the timescale of days: I'm part of the Finnish reserve. When I was younger, if called upon, I would have been deployed in my pre-assigned, pre-planned, military position of defending a major branch of nearby industry within 48 hours, with a task force of some two dozen people fully geared up and under my command. There's literal warehouses very near population centers to quickly hand out weapons to the early response groups. Main force activation happens on the timeline of a handful of days, staggered largely based on age so youngest most combat-ready first and older people in coming weeks or months. Every reservist has been pre-assigned a role and where to show up in case of crisis (whether they know it or not).

Maybe it's intended to help with calculations. For comparison, the German BBK recommends a emergency stock to last ten days.