Why won’t companies ever raise their current employees wages, but they’re willing to start a new guy off with a higher salary for a similar role?

It seems to be an emerging trend. Companies absolutely refuse to raise the salaries of their current employees. Yet at the same time, they have no problem in offering a much higher salary to a potential new hire for a role the same as the current employees. It's cheaper to retain staff than to hire new ones and train them. Don't get me wrong – I'm not saying companies should lowball new hires. But they shouldn't ignore their veteran workers.