The WC doesn't prevent hiring directly in a senior union position, but it mandates that any internal union employee with the requisite knowledge and the ability to pass the relevant exams comes first, in order of seniority.

To ensure candidates are qualified these exams are difficult and change everytime, and success threshold are high. Quite a few older but sadly less qualified employees with a ton of seniority tried them multiple times over the years, failing every time, so it's a system that works. Often you'll have nearly a hundred applicants and the position goes to the 75th guy in order of seniority. Its a fine balancing act, the union also wants these positions staffed by very skilled people but we wouldn't want hard exams to be a way to bypass us and hire externally.

So in order to get us to accept very high difficulty threshold, they also agreed to let us write half the technical tests. Which we promptly packed with questions about in-house tools or obscure systems you likely never heard of unless you worked at the company for awhile (we didn't push it to the point where non-union departments had no chance though - we wanted within the spirit of the contract. We also grade the technical portion.

So the one time -every single applicant- in house failed a particularly difficult set of tests for an opening, HR were allowed to offer these positions directly out of house, but you can guess what happened. Huge list of candidates, but nobody external could beat the exams either. Some applicants even got angry at HR for wasting their time once they saw the tests. Which was particularly funny. So of course we immediately suggested toning down the exams a notch together and reopening the position internally, which worked fine and filled the position with someone qualified and experienced.

It can seem like a hassle, but these are jobs people usually leave once they retire, die, take up a middle-management job or have an incurable mental breakdown. Turnover rate is insanely high. So both the company and the union are invested in ensuring their respective needs are met in the hiring process.