Thing is that there are already tests out there that test for programming ability but they tend to either be very academic and mathematically focused like proje

Thing is that there are already tests out there that test for programming ability but they tend to either be very academic and mathematically focused like project euler or on trivia related to specific languages or technology. When you have a working team, chances are that you already know most peoples strengths and weaknesses as it relates to the job function, so a test isn't so useful in finding areas for improvement. When hiring someone new you are either looking for people with the same skill set or a complimentary one. The problem remains that the best hires are probably in enough demand and know that they can get a job without having to take your test, especially if they need to pay to take it, so the people most likely to try and get a good score will be the worse hires who have a strong incentive to try and game the test which would be easy if the questions don't change often. Also, if the test were proven to be a reliable assessment of skills across the entire industry then people who scored highly on the test would quickly start asking for much higher salaries. But what employers are looking for is to get a lower salary on employees who have a good skill match for the employers specific problems even if the don't score well in some areas. So there is a reason for employers to develop their own tests.