They don't put all their Bitcoins in Cold storage for the same reason you don;t put all you money in a safe deposit box. If you want to have access to it so you can use it, it only make sense that you would keep it out of cold storage.
What I should say is, you don't need to put all of your bitcoins into cold storage. You can keep the coins that you want to use in a hot wallet. If you own 10+ bitcoins do you need them all out to use?