True, it's much simpler. It's also much more interesting when you start having a huge codebase, and you want to refactor/add a feature/fix a bug, because you don't need to set and verify the whole input/output of your application but only the one that does the logic you're interested in.

It's also simpler to use unit tests for verifying all possible inputs and outputs of a component.

But, as others have said, unit tests alone are necessary but not sufficient.