Why Is There No One Else in the Apocalypse?

"The apocalypse suddenly begins, but why are there zombies and no one in the apocalypse!!" Li Fei wandered alone among the ruins, confused. The protagonist survives the apocalypse alone, cleans up zombie monsters, farms and develops alone, and builds a lonely fortress. There are no supernatural powers or fantasy. The protagonist only relies on his own knowledge and physical strength to fight. Supporting characters may appear later. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.