An API is an official interface to connect to a service, usually designed to make it easier for one application to interact with another. This is usually kept stable and provides only the information needed to serve the request of the application requesting it.

A scraper is an application that scrapes data from a human readable source (i.e. website) to obtain data from another application. Since website designs can update frequently, these scrapers can break at any time and need to be updated alongside the original application.

Reddit clients interact with an API to serve requests, but Newpipe scrapes the YouTube webpage itself. So if YouTube changes their UI tomorrow Newpipe could very easily break. No one wants to design their app around a fragile base while building a bunch of stuff on top of it. It's just way too much work for very little effort.

It's like I can enter my house through the door or the chimney. I would always take the door since it's designed for human entry. I could technically use the chimney if there's no door. But if someone lights up the fireplace I'd be toast.