If Praising Employees For Doing A Good Job Seems To Be A Fairly Easy And Obvious Motivational Tool, Why Do You Think Companies And Managers Don't Often Do It?

It's all about incentives. If they thought you'd do a good job for them simply because they told you what a good job it was, the wouldn't bother to pay you. As it is, saying "good job" is something most people don't think of, because it's seen as kind of pointless.