Psychometric tests are, unfortunately, a very blunt tool. As noted above, they can be practiced...ensuring skewed results.
But...they cut recruitment costs significantly when there are thousands upon thousands of suitable applicants.
The tests will predict outcomes with reasonable efficacy over a large sample size but sometimes get it very wrong.
This is where a competent and well resourced recruitment department will use a human touch to rule in (or as importantly, out), candidates that otherwise did not score within the required parameters.
The tests will always keep some excellent pilots from joining an airline and unfortunately allow some mis-hires through the front door!
But that's life in general, isn't it? No one said it would be fair!