Here's a good counter example:

https://www.hec.usace.army.mil/software/hec-ras/downloads.as...

Back when I did work on hydraulic modeling, HEC-RAS was the gold standard for open channel flow. It is only available as a pre-compiled Windows executable.

I had a ton of parameterized simulations to run and would have loved to have gotten my hands on the source code in order to run it on EC2 Linux spot instances and to mechanize the parameter inputs by cutting the GUI input forms out.