Traditionally, most neural network architectures have been implemented in C/C++ - for performance reasons. But ML researchers are not hackers, for the most part, and Python has the lowest impedence mismatch for interfacing with C/C++ of all the major languages. Julia was popular for a bit, but now Python is dominant. Programs tend to be very small, and not modular - so static type checking is less important than it would be in picking up errors in larger systems.