They get an unspeakable number of applicants and finance requires horse like work ethic, a high tolerance for boredom and detail orientation. A high GPA is a strong signal of all of those and also has a very high correlation with g/IQ/processing speed.

Work sample tests might work better but at least in the UK they do multiple rounds of structured interviews followed by free form interviews after filtering by class of degree/university prestige. I bet their false positive rate on hiring is very low even if the false negative rate is high.