I say it's all about data flow and composability, if it's pretty much always in one direction (modular tree structure/architecture) then you just don't need all these "patterns"...

What does Go’s simplicity have to do with dependency injection?

In my experience, following Go's philosophy of simple solutions eliminates the need for complex solutions such as dependency injection.

How do you unit test your extremely complex projects if your business logic carries the additional responsibility of creating objects?

I write modular code that accepts interfaces so I can test the components I want to test. The vast majority of object creation happens at initialization time, not in the business logic. For the projects I've worked on, that would be true with or without DI - I don't see how that's relevant.

perhaps your “extremely complex projects” wouldn’t be so extremely complex if you practiced dependency injection?

When the CTO says, "Make it distributed and sharded," I do what I'm told, but that is an intrinsically complex problem. The complexity is in the overall behavior of the system. If you zoom in to the individual execution units, the business logic is relatively simple. But the behavior of the system as a whole is rather complex, and DI isn't going to change that.