They do. Aren't you paying attention to your ever-increasing bandwidth allocation, ever-increasing disk storage quota, new features that are regularly added at no additional cost to you, and the "forever" price freeze for existing customers?
You'd think you would want to keep your hosting, wouldn't you? Of course, to renew, you would have to actually pay something (more than a couple of bucks a month) for your service. Sheesh!
The best "incentive to renew" is the price, value, features, and service of your alternatives. If you find a better total deal for you than what Dreamhost offers on it's regular priced plans, you should take it.