> why don't they just provide an explicit way of passing values instead of hiding them in the context?

Hiding them in a context is the explicit way of passing values through oblivious third-party libraries.

In some future version of Go, it would be nice to just have dynamic scoping. But this works now, and it’s a good pattern. The only real issue is the function-colouring one, and that’s solvable by simply requiring that every exported function take a context.