What Michaelbaubles said. A no deal means we are hanging out with our asses in the wind with absolutely no idea what comes next. And that means that moving goods in and out, services, people etc becomes difficult. Sure, for example we can continue producing food products to current standards and hope that is all okay, but what happens when we actually get to the border and no one knows what tariffs to employ? Even if everyone involved knows that the food safety standards have been employed in exactly the same way, is the documentation LEGAL with a no deal. Just because we SAY it's all exactly the same, it doesn't change the fact that from a purely LEGAL perspective, it's not.
The point is that Brexit isn't about small time issues and politics. Just because you say that everything is the same on the quality of your goods/people/services and you produce your own evidence, it means nothing if that evidence has not been legally recognised.
I think this is the bit people struggle with. There's this kind of unspoken view that "common sense" would prevail. But it doesn't matter how sensible and obvious things are, nations are based on a complex and detailed legal system and no one is going to let that slide because the moment you do, you open the floodgates.....
To continue my example above, legally, all this documentation I've been talking about, you could argue, is exactly the same so why does it matter? But once we leave the EU, that farmer has all this documentation, but there's no built in legal process that allows, for example, the EU to CHECK that the paperwork is being processed and handled correctly. So then what happens? What if it slacks or someone tries to work around it? The EU has no recourse and no way to know that the paperwork, which looks and feels and says the same things as it always did really IS the same. Because, again, this is how legal systems work. There are processes and fail safes etc built it.