Because there’s other things that you have to do. When I first played Triple Deluxe, I didn’t expect to get 100% completion right after I finished the story mode. There were sub-games I had to complete and I had to collect one of each keychain.
With the exception of King Dedede's stage, after defeating a boss, the player be able to re-enter the stage to play a star-catching mini-game. Depending on the boss level, Kirby must evade any harmful objects while catching all the stars; this is required to achieve 100% completion. If Kirby gets hit once, he will be taken out of the level immediately and the stage's entrance will be closed, only opening again when the player beats any other level.