A lot of people who have problems with various recruiting techniques seem to assume a universe where nobody is dishonest. You can call for references but in many cultures people do not want to be mean and will say nice things about you even if you were a terrible employee. People will also lie and scheme and it can be difficult to really dig into someone's professional career to know if they are serious and good people or not. This is where some official record like a GPA can be useful, or a live coding interview where you get the opportunity to at least prove you are not a total faker.