As someone on the other side of this, recruiting for a big bank, it was depressing.
The reason we defaulted to the GPA was exactly because it was so easy to do the work.
Let me explain.
When we visited a school (usually Columbia or NYU for me) we had 100s, usually over a 1000, resumes. When I talked to the students, they were all bright, smart, interesting and motivated. From their resumes I got the impression any of them could do the job. And yet we had to pick maybe 10-30 from the school for that summer. (Keep in mind we recruited from many schools for many different positions).
So you have 1000 resumes and they all seem smart and interesting and you have to pick 10. What do you do?
The GPA is a great way to numerically and “objectively” make a cut. Then when you see a typo and maybe you can exclude that one and so on.
The reason you have to rely on these non predictive elements is exactly because everyone signals well on the predictive elements.