But in a lot of cases you can't know all the dependencies, so you lean on the community trusting that a package solves the problem well enough that you can abstract it.

You can pin the dependency and review the changes for security reasons, but fully grasping the logic is non-trivial.

Smaller dependencies are fine to copy at first, but at some point the codebase becomes too big, so you abstract it and at that point it becomes a self-maintained dependency. Which is a fair decision, but it is all about tradeoffs and sometimes too costly.