Not only "climate models"... a large chunk of scipy is just a thin Python layer over decades-old Fortran code. That many physicists chose to use the real thing instead of the fisher-price interface speaks in favor of them.
Actually, as a user of the fisher-price interface, I'm glad that it can bind to C and FORTRAN libraries, so my numerics are based on the highest quality code.
I'm also a very happy user of the dumbed-down interfaces. But I find it strange when my fellow users are surprised or even horrified at the fact that some people still write and maintain Fortran and C code. Hell the Python interpreter they use is written in C! But apparently if you write in C you are some sort of old man yelling at clouds.
Gonum wraps the same code, from what i could tell fortran seem to have a neat way to handle more specialised number systems like dual numbers and hyperbolic numbers.