Well I mean you've answered the question right there. The salaries and benefits for the current employees are a sunk cost and are getting paid no matter what. So having them spend a few months talking to people isn't an additional cost in the accounting sense of things. The minute you add "paid trial task" you're talking about a (usually non-trivial) expansion in budget. Where does the money come from? How many of the devs on that team are going to be happy with smaller bonuses, smaller raises, so they can pay someone to write code they're going to reject?

Maybe this is objectively The Right Way to hire developers, but good luck convincing the accountants of that, and good luck convincing your current developers to pay for it.