> When you make a creative work (which includes code), the work is under exclusive copyright by default. Unless you include a license that specifies otherwise, nobody else can copy, distribute, or modify your work without being at risk of take-downs, shake-downs, or litigation. Once the work has other contributors (each a copyright holder), “nobody” starts including you.
It doesn't even have to be there, you could state what the license is on a website or in an e-mail. Sometimes you can find it in the header of source files (as seen here as well). Having a "LICENSE" or "COPYING" file in the root of the repository is just a common pattern made even more common by all the tools that can consume it automatically (including GitHub's UI).
Relevant: https://github.com/orgs/community/discussions/82431
> When you make a creative work (which includes code), the work is under exclusive copyright by default. Unless you include a license that specifies otherwise, nobody else can copy, distribute, or modify your work without being at risk of take-downs, shake-downs, or litigation. Once the work has other contributors (each a copyright holder), “nobody” starts including you.
https://choosealicense.com/no-permission/