Even if the Apostles did remember everything Jesus told them, they didn't record it all. John 21:25 states, "And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written."
Perhaps you think that the majority of the things Jesus said and did were not important enough for them to have written down, and that they did record everything He said that mattered. Look at the length of the four gospel accounts of His life. Do you honestly believe that everything He said or did that mattered was recorded? He must have wasted a lot of time, if that's the case!