They post a day or so beforehand, and with how many sales they do that is fine. As another said, if you feel it is worth buying at the RP price, buy it. If not, don't. Sales are a convenience, and having a set schedule doesn't really do anything to help them. Nor do many people complain about the lack of two weeks advance notice. And if they DID give that notice, people would then just go "WHAT!? I bought this a day ago, but it's about to go on sale!?" and we'd be in the same place.
To make the retail analogy correctly(you took a car sales thing, which are all farces when it comes to final sale price), you are not usually warned when an item is about to have a price drop. They just do it. Warning everyone is not a wise business strategy. Internet wise, STEAM and Impulse both do not warn you a game is going on sale in 2 days, they just do it.
In short: If you're that bent out of shape, stop buying things on the internet.