It wasn't the default. People used to eat seasonally available food. Similar to keto in some ways but very different.
In the summer there are leafy vegetables, eggs, and animal milk. All can be fermented for storage.
In the early fall there are fruits and root vegetables for harvest and storage. These can be preserved by fermentation or drying. Nuts can also be picked now
In the late fall almost all domesticated animals are butchered for the winter. Wild animals are also now in season for hunting. Meat is smoked and salted or fermented for storage.
I'm the winter and spring you eat what you have saved all year.