Why aren't MBB (and other firms) upfront about the amount and type of of preparation required to be successful at interview?

A number of comments here:

1) Firms underplay it because they don't want candidates that will be good at interviews as a result of countless hours of practice - they want candidates that demonstrate the key skills/qualities that consulting firms look for. To demonstrate the core of these, 5-10 hours of proper practice was typically seen as enough. Firms don't want to miss out on potential strong candidates just because they were not aware about consulting 6 months before interviewing and could dedicate countless hours to prep - so major firms at target schools often have case-workshops to try and limit this bias.

2) In a way, it is due to sites like Victor Cheng and PrepLounge that has led to the requirement for most people to do many more hours of practice. As everyone has access to more and better material, the average quality of interviews increases - so firms have to be more selective, and the "passing criteria" have become more and more demanding.

3) Finally, on your point about DCF - I think this is a huge misconception. If you are applying from undergrad or from a non-business background, you will not be expected to understand DCF, NPV, etc. Part of the reason people think this is expected is because most case resources come from business school (MBA) case books - where students learn these topics, and see firms might choose to test them on their knowledge. However, I can guarantee that if you are applying to MBB out of undergrad, you will never be expected to know these technical terms/methods, or be penalised for not knowing them. The only exception might be if you are applying to a very specific role within a company (e.g. M&A advisory) where e.g. NPV might be particularly important.

Hope this clarifies some of your questions.