A shorter answer is greed. Public companies leaders don’t get bonuses and will get fired if they miss a profit opportunity so take most new opportunities and hire away. As soon as things don’t pan out, they then will lose their bonus and get fired themselves if they don’t show rich investors they’re cutting back–amd they outsource the consequences.

Something to consider when evaluating job opportunities are layoff history-- it’s pretty easy to find and gives you an idea of the company’s stability and internal judgement and practices. Some companies do layoffs every year, “time to get my q1 bonus” and others plan, execute and consider carefully around their employees.