I work in a field where lots of code is written, but hardly ever tested. This is because we are foremost scientists who try to solve problems with code. The few coding courses we had, focused on the basics and many have never heard of git, unit testing, clean code after graduating. Many haven't even heard of those during their PhD...
Maybe its better now, but 10-5 years ago we did not have any mandatory courses which cover those areas.
Often the software solves differential equations numerically. In many cases PDEs with many feedbacks going on.
Think of weather predictions, chemical reactions, atmospheric models and so on.
So now my questions, would you trust results of a complex software with many hundreds or thousands of functions without a single unit test? If there are tests then they are rather high level, like to check if the results stay the same with the same input or if the results of a very simple case fit an analytical solution.
Even if you know that the numerical solution of the equation is sound, based on some years old publication, would you trust the model to make predictions? Would you trust it if it can cause billions of damage of even loss of live?
On a side note, often these models are compared against each other with the same simplified inputs.