It's mostly related to the fact that Elixir code is very terse and easy to understand, and that if you put 100 devs on a singular Phoenix web/API project codebase they will be stepping on each others' toes all the time so they need to kind of take turns if they don't want the codebase to become a complete mess.

Thirdly, Elixir devs are on average quite senior in general (as they are usually refugees from other stacks) which lends itself to one-man armies very well in my almost 8 years of experience working with Elixir.