Screening / testing is for the benefit of applicants and the airlines.
Many self sponsored cadets, starting their career with smaller operators, will only have an interview for their selection.
I have witnessed two pilots lose their jobs in my short time in aviation purely because they could not handle the pressure of the cockpit, and may therefore react badly in an emergency (anxious introverts rather than stable extrovets perhaps!). Some kind of selection / screening process right from day one would have saved the pilots, and the airline, a whole lot of time and money - and heartache.