There isn’t much justification given, but see section [5.7.6][1] in the manual, but I think the following answers your question:
If you do not create any schemas then all users access the public schema implicitly. This simulates the situation where schemas are not available at all. This setup is mainly recommended when there is only a single user or a few cooperating users in a database. This setup also allows smooth transition from the non-schema-aware world.