Well, that’s just it. Orbits can EASILY change. A gravitational tug by a planet/another object, impacts, ect. Sure orbits don’t change on their own by a significant amount, but there are MANY ‘things’ that can make changes happen.
We don’t have a thorough accounting of what’s out there. We don’t know what we don’t know about. No one had any IDEA that close call over Russia a few years back was going to happen. If that thing would have went straight into the Earth over a populated area, it would have been catastrophic. No one saw it coming.
Our planet WILL get hit by a ‘big one’ eventually. We can only be comforted by the fact that our Solar System and the Universe is a dammed big place, and it PROBABLY won’t happen within our lifetimes. Humans most likely won’t even be here when it happens again.
Apples versus truckload of oranges. The Chelyabinsk meteor was an apple. It is true that nobody saw it coming. The largest intact piece would have destroyed a building, but not caused a catastrophe. The meteorite was 20m.
The newly discovered object is 400m. Its orbit cannot easily change since it is being tracked and interactions with other objects can be calculated. Even if it hit a unknown 20m meteor like the Chelyabinsk meteor, the path won't change much and can be easily reanalyzed.