Not entirely true: I am an astrophysicist, and I can say that this practice would be extremely useful for my discipline as well. In cosmology (my field) we use codes that are often extremely complex, yet they are not usually released together with the papers based on their outputs. This puzzles me, as last year I attended a conference on informatics and astrophysics (ADASS), and one of the talks showed that releasing your codes as open source increases a lot the chance of your papers being cited (expecially if you advertise your code using services like ASCL [1]).
The only reason I can think of this unwillingness to publish codes is the fact that these codes written by physicists are often extremely unreadable: very long Fortran routines with awkward variable names, no comments, no test cases, no documentation… Once you get a result from your own software, you get more satisfaction if you publish a paper with the results than if you polish and publish the code.
IMHO this is bad science, but it is difficult to change this way of working: cosmology is today done by large collaborations, not by individuals, and if you propose to make the codebase developed by your team public, this idea is usually not welcomed by your co-authors.
