The problem is that hiring someone to "test them on the job" is not free, in fact making a bad hiring decision can cost hundreds of thousands of dollars (onboarding, training, salaries/benefits, disruption to the team, cost of poor quality, severance and rehiring, etc). The outcome also often falls on the shoulders of the hiring manager, as a result most managers are generally very conservative when making hiring decisions.
You wouldn't trust a contractor to do a $100k reno to your house without looking at their reputation