Because now [you or some other person] can no longer enjoy this dead person's company, and them dying reminds you of your own mortality which scares the normals. Everything normals state about death comes from selfishness, they even project their own selfishness onto those who commit suicide calling it as such, yet their reasoning is always what I wrote above.
If you want to truly understand normals just take everything from a "does this make me feel good?" perspective, they are all unthinking hedonists at their core. Once you can do that you no longer feel confused when they inevitably end up contradicting themselves, they have no internal logic or being really. They have no pride or ego, no virtue or sensibilities, there is nothing "under the hood" so to speak. Only an extreme drive to "feel good".