Because there are some basic facts.

The EU, overall, does something like 6%-8% of its trade with the UK. At higher cost/effort, that percentage would drop some, but it would not be devastating.

The UK does something like 45%-50% of its current trade with the EU/EEA. An increased cost there will have a significant impact on the UK economy as a whole.

The main problem many people overlook is that the bulk of the UK export trade, so what the UK sells to make money is services, and they import goods. Now, trade in goods is relatively easy to understand. Services are much more problematic. When a UK banker, lawyer or other service provider loses its certification (as a result of a No Deal Brexit, the UK drops out of all EU rules and regulations), that banker or lawyer can no longer operate in the EU27. That is why so many banks have been moving their businesses to Dublin, Amsterdam, Paris, Frankfurt ..