As a returning player, I'd ask a contrary question: why the hell mobs can now spawn in your face while you look at them? Is there any lorewise explanation other than toxic players? All the illusion of the world persistence is lost...
That's the golden rule of almost every game to trick trick our minds to believe that for everything there is a cause, and stuff don't just appear randomly – the weapon crate is an airdrop, or an enemy was there behind the corner all the time. The Giant Spider was small some time ago, as it grew from an egg, but maybe it was hidden somewhere between the narrow corridors of the dwarf mines and it came to claim a hunting territory of the previous incarnation that you've just killed... Right now, they're just continuously teleported from another plane of existence.