Hostile mobs do not despawn, no matter how far you are from them

Hostile mobs do not despawn, even after I've walked hundreds of blocks away from them. There are mobs that spawned on my first days on the map near the world spawn that are still there after over 30 hours of play thousands of blocks away. Visibly reduced mob spawning at night due to the ammount of drowned on nearby chunks (sometimes 20 or 30 inside a single water hole).