He's left behind because god f**ked up and somehow because MC is born with a powerful concealment ability, but really, god f**ked up. Then he's stuck on earth alone for 1000 years instead of 10, because god f**ked up. Then the monsters on earth are too strong, because guess what... God f**ked up. Then the traps don't work, because... God f**ked up. Then some dude wants to invade earth, and he's able to do so because... Yep, god and his angels f**ked up. God f**ks up > Hey MC fix this sh*t > Big reward > rinse repeat.