Because you don't need PhD for those jobs. But as a PhD you would be asking a higher salary than a BS(CS). So its simply more efficient for the company to hire

Because you don't need PhD for those jobs. But as a PhD you would be asking a higher salary than a BS(CS). So its simply more efficient for the company to hire an experienced BS(CS). But with a PhD, you can target jobs that the employer feels are too complex and demanding for a simple BS