Look back to the example of 1000 applicants for only 200 positions. The airline can mandate that applicants have a certain GPA, and suddenly they weed out 150 people. Then the airline can weed out people with multiple checkride failures, that might take another 150 people out of the running. Now they are down 300 applicants by only having to look at two numbers.

A 4.0 GPA doesn't make someone a good pilot, just as 3, 4 or 5 checkride failures doesn't necessarily mean that a pilot is bad. Its not fair. But they are both numbers that airlines can set arbitrary standards and weed out a bunch of applicants easily. Airlines aren't the only employers that make these kinds of requirements.