I agree, although I wouldn't describe the overall issue as developers not testing to ensure reproducibility. The reason most builds aren't reproducible is that build reproducibility isn't a goal for most projects.

It would be great if 100% of builds were reproducible, but I don't believe developers shouldn't be testing for reproducibility unless it's a defined goal.

As generalized reproducible build tooling (guix, nix, etc.) becomes more mainstream, I imagine we'll see more reproducible builds as adoption grows and reproducibility is no longer something developers have to "check for", but simply rely upon from their tooling.