worlds get deleted after a random crash

i can remember 2 instances when the game crashed and the world gets deleted. When the game crashes and i go back in Minecraft, the world i was in when the game crashed got deleted. i don't have a crash log for this, but i don't even know how this happens. This usually happens with old worlds. I made a world on July 11, 2016, and the world got deleted on June 8, 2018.