Cain and Abel were the children of Adam and Eve, the first humans, so until Cain killed Abel, none of them had ever witnessed or experienced human death before.
Cain was the cause of the first death in human history.
For Cain's sin, God banished him from his land to live a nomadic/fugitive lifestyle, and bestowed him with a body that could not be killed, which can be interpreted as immortality.
Leprosy was rampant in Biblical times, so despite this immortality, his body could still succumb to disease. He may have even been patient zero and started the spread to others he passed by in his wandering. But he still could not die from it.
He roamed the world with his undying body rotting from the outside in, unable to stop to rest, unable to end his pain, until he was finally no longer flesh, only bone.
Cain was a farmer who grew grasses such as wheat, so it would make sense for him to have a scythe in his possession. He wears the cloak to hide his putrefying corpse, and eventually his fully skeletal figure.
Now he, as Death, is known to bring souls to the afterlife after their passing, a new kind of harvest in a way. Maybe he hopes to atone for his past actions. Maybe he enjoys having this be his new purpose.
He was the being who caused the first death, so his existence is now forever tied to the dead.
DISCLAIMER: I'm not religious, I'm not going through the Bible to cite all my sources, this is just a fun story idea that i couldn't get out of my brain and couldnt find anyone else talking about. thank you for coming to my ted talk