There would be no room for error if we just put the libraries in with the project as files instead of adding all these extra steps. Nobody seems to like this simple, bulletproof method anymore for some reason though.

That's exactly what a package manager does

A package manager is a whole separate program with config files that adds an extra build step and works like a black box. I mean just having the libraries entirely present in the repository. If an update is needed then someone pastes it in and commits it. This also lets you organize how you want.

That's often called "vendoring" these days.