Pilots need to know a huge amount of information. Back when learning how to fly meant buying physical books versus using PDF downloads or apps or even CDs or DVDs, one could physically see how much one was expected to know. Stacked together, the basic textbooks for private/commercial IFR, multiengine flying, the FAR/AIM, weight and balance, weather—I’m sure I’ve missed something—came to thousands of pages, not to mention various pilot’s operating handbooks.
Source: aopaWhat to memorize?