Why is it ridiculous?
Time for a car analogy. You can spend as little as possible on the cheapest car you can find and it'll get you from A-to-B. You can also spend hundreds of thousands of dollars on a car that'll do the same thing - the difference is in the user-experience. People spend more on a car in order to make going from A-to-B more enjoyable. Maybe it goes faster, maybe the ride's smoother, maybe they look better doing it, maybe it has some extra gadgets, but in all cases people are spending more to improve the user-experience. The same goes for keyboards - and most computer components. The more you spend typically makes for an improved user-experience.
Now, your avatar leads me to believe that you think $100 for a case is reasonable? I won't dispute that, but lets compare it to a $50 case. What is that extra $50 getting you? Maybe some slightly better fans or a cheap fan controller, perhaps it made your rig easier to put together, but how much is that extra $50 getting you right now, as you read this? Not a whole lot. A case does an important job that's worth doing, but it doesn't add much to the user-experience. It might look pretty, but you're looking at the monitor right now, not your case.
Now lets consider the keyboard. Aside from the monitor and maybe the mouse, it's probably *the* most used and utilized part of your computer. The extent of your interaction with your case is pressing the power button when you switch on a computer, but a keyboard is used constantly, be it writing passwords, e-mails, forum posts, URLs, search queries or controlling games. So, this time, lets compare an $80 keyboard to a $50 one...What does that extra $30 get you? Mechanical switches. It might get you a slightly better appearance and it'll definitely get you significantly improved build-quality, but at the end of the day it's primarily getting you a keyboard that feels worlds better to type on...A keyboard you'll use day in and day out, an ever-present part of the user experience.
Given a $50 computer case and keyboard, with the choice of spending an extra $50 on either? Why would I choose the case?