This is bit confusing but due to the separation of payments related code into the separate app, that app is now a dependency of ERPNext (for use in POS, E-commerce etc etc)
We need to update installation docs related to this to clearly reflect requirements. cc @ritwik
btw bench also has --resolve-deps flag that automatically resolves dependencies when installing new apps. You might have seen this message when doing get-app:
Ignoring dependencies of https://github.com/frappe/erpnext. To install dependencies use --resolve-deps