If astronomers can see stars as they were thousands of years ago, why can't I see my grandmother or the CIvil War?

Because the photons that made up those images long ago, were either mostly absorbed by the walls of a room, the atmosphere, or if they managed to escape the Earth, have now thoroughly blended into the much vaster stream of light emitted by our Sun which sweeps past the Earth by the quintillions of photons per second. Your grand mother's image is thoroughly lost in the solar flux.