It does, but express doesn't tell it directly in the docs.

Instead http://expressjs.com/en/4x/api.html#app.listen sends you to https://nodejs.org/api/http.html#http_server_listen which also won't give the answer but sends you to https://nodejs.org/api/net.html#serverlisten which at some points finally documents that default is 0.0.0.0.

Welcome to the JavaScript ecosystem :/