[drjesusphd]

It's usually highly-parallelized Fortran ran on the world's largest supercomputers, utilizing thousands of CPU cores. There are several codes like the ones in the study above (google "gyrokinetic equation solver"), and somehow more pop up year by year. So it's not a matter of sunk costs.

And yes, GPUs are increasingly being utilized, depending on the algorithm. But, GPUs aren't magic; they don't speed up every kind of problem.