Different budgets. Raises are continual, ongoing costs but the cost for hiring a new person can be considered one off costs, even though you are performing the "one off" task repeatedly
Some managers also seem to only respond to crises. Being down an employee is a crisis. Making sure that your employee gets an appropriate raise isn't, especially since most people aren't going to leave their boss know that they are looking for another job
Re point #1, I simply don't understand how, long term, it is more cost efficient to hire and train someone entirely new than it is to build a culture of valuing current staff. Maybe I'm naive? Maybe it is difficult in ways I don't perceive?