There is no NEED for any kind of border between the EU and the UK. All our laws and all our regulations and all our tariffs are 100% aligned and identical. The need for a border is an invention of the EU and the reason for this invention is good subject for speculation.

Speculation 1. Based on the fear that other members may be tempted to follow suit and leave, the EU wants to be seen to be punitive towards leavers.

Speculation 2. The EU knows that a significant number, although not majority, of UK citizens do not want to leave. This large minority (actually a majority of MPs) will do their best to prevent Brexit even to the point of damaging the UK. The EU therefor gives as little as possible and asks as much as possible knowing that remainers will attempt to sabotage a successful exit. An example is the remainers insistance that 'No Deal' is taken off the table. No Deal will damage the EU far more that the it will damage the UK. It should be a really good bargaining tool. The remainers do not want us to have such a good bargaining tool. Also of course 'No Deal' actually means leaving. Every bit of any deal we do will tie us to the EU. The softer the Brexit the greater and tighter the ties will be. No deal means we leave and the remainers have lost completely. Bring it on.

Speculation 3. The EU is a psychotic and nonredeemable political experiment that puts an inflexible ideology before the best interests of it's member states and their populations. They would do this wouldn't they?