Random question: Why are we still using mostly GPUs for computation rather than CPUs custom-designed for ML tasks?
GPUs just work very well when you have a a lot of data and you are able to run the operations on the data set in parallel. Machine learning seems to fit this model quite well which is why you see many GPUs used in this field. Other things that take advantage of parallelism would be graphics and crypto-currency mining.
[0] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.159...
ML might be a bit of a moving target though.