Because he is reaping the souls.

The scythe as an artistic symbol of death has deliberate agricultural associations since the medieval period due to the importance of farming and seasons. The tool symbolizes the removal of human souls from their bodies in huge numbers, with the analogy being to a farmer (reaper) cutting through large swaths of grain crops during harvest. https://en.wikipedia.org/wiki/Grim_Reaper

Edit: Death personified as someone harvesting is a very old metaphor:

For death is come up into our windows, and is entered into our palaces, to cut off the children from without, and the young men from the streets. Speak, Thus saith the LORD, Even the carcases of men shall fall as dung upon the open field, and as the handful after the harvestman, and none shall gather them. Jeremia 9, 21-22