They specifically want to economically encourage bringing the house with you to get the cheapest rate (total commitment); or encourage building a larger house than you might otherwise have. A continuous function of price reduction per unit, rewards everyone as they go; Amazon very specifically doesn't want to do that (users gain benefit at every step, and are rewarded just for being at any given metric, encouraging no particular scale or usage).

The price points are supposed to be mental carrots, in other words. It can of course be debated which approach is better, but I have to suspect Amazon has a lot of data on scaling and pricing from retail behavior.