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.
It's also because the cost of making things reproducible is still too high.
We have the tooling, but it still takes a bit of effort from the developer's side to integrate those into their CI pipeline.
Eventually we will get to a place where this will be the default. It will be integrated into day-to-day tooling like `cargo release`, `npm publish`, ...
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.