Why are caskets hinged in the middle?

Like this.

Why not just open or close the whole lid?

Oops GQ - no jokes in post 2.

A major purpose of caskets is displaying the remains at a service. A hinged lid allows them to put only the good half on display.

Plus it promotes the comforting illusion that the dead person is only “sleeping”. The half-open lid makes the deceased look like they’re tucked in a bed.