I mean it's not even a reach for a suitable analogy.
Picture an airport with two queues. One for eu passport holders, one for the rest of the world. We've all seen it and experienced it: the eu passport queue involves the most cursory of glances, the checks for the rest of the world are more thorough and time consuming. Even with this the eu queue is sometimes very busy at various hot points of the day.
Now imagine this same process for (hundreds of?) thousands of lorries, cargo planes, cargo ships per day. Now imagine there's only one queue and its the rest of the world one. This is what we will be doing to ourselves on a very basic level if there's no deal.
We have two options really, to just wave shit through without checking it (meaning all sorts of unsavoury / dangerous stuff could be brought into the country) or to thoroughly check everything which would be incredibly time consuming and cause massive holdups. I suppose we could check only the bare minimum of goods, but again, this would be an opportunity for people to bring people and things we don't want into the country and is the exact opposite of taking back control of our borders.