Because even though the virus is detectable on surfaces after a certain amount of time, it degrades further and further hour by hour. The risk of there actually being enough virus to infect you is negligible.
Then there's the fact that if you wash your hands as often as you should be, even if there was enough virus on a package to cause infection, you'd kill it with a hand-wash anyway.
But do we know just how contagious this viris is yet for sure? It's brand new, it was only named in January and scientists are obviiusky still studying it and finding more about it every day
For example the measles virus is now known to be highly contagious. It can be caught by being in a room hours after an person infected has left. Even if that infected person didn't touched anything in the room, or even sneeze or cough. I believe the common cold, a coronavirus, can be caught this way too
Yes it might be over the top to clean shopping, or to change outdoor shoes and clothes in the hallway. But surely isn't a few minutes of time worth it to reduce possible risk. Lets face it, a lot of us have a few minutes time to spare anyway and the less risk taken the better, for everyone