Most code starts out as a monolith. This was my experience as well while working on a self-service application for a big telecommunication provider. Idea was to have a separate application where we will expose a lightweight REST API for our modern frontend (Angular 1 at that time). This consumer services application was supposed to be a bridge between CRM and SOAP powered web services and a lightweight client application that we developed using Angular. Without knowing full boundaries of our domain or the extent to which it will grow we started exposing REST services and it seemed like a good approach.
So what is a monolith?