Ejabberd runs with a lot of legacy. Essentially the first build tool that could pull deps in Erlang dates from 2007-2009, and it took another few years before more modern tools came to be in what their current form is.

All these bigger projects tend to have a bit weirder idiomatic build setups because they had to haphazardly reimplement bits of (at the time) yet non-existing build systems by hand.