Because NextJS docs say so:
https://nextjs.org/docs/app/api-reference/file-conventions/metadata/app-icons#favicon

Im sure you can put them in another folder and use a function to tell NextJS where they are but i dont remember the name right now.

I know that the docs refer to it but i cannot understand the purpose of it, when every react project holds all the static assets inside the public folder, the fonts folder too....

Probably its just a design choice

Because different routes can have different icons if you colocate them that way.