Culmination of 10 years of Marvel movies, Avengers: Endgame acknowledged its hard-core fans that have been following the franchise and picking up on even the minute of details. Every other scene is like an allusion/homage to previous movie event/clips. If you haven't watched the previous movies, or have just watched them for the sake of watching Endgame, you haven't gone through the same emotions a Marvel fan has, and you won't properly enjoy it. Time gap b/w the films is really important, so as to let the events of the movies sink in. I've got several friends who just watched the whole MCU 5 days before release of Endgame. If you're one of those, I'd not be surprised if Endgame didn't live up to your expectations.