I really cannot fathom how people don't understand the facts.
1. The UK has signed an international treaty (The Good Friday agreement) with Ireland which states there will never be a hard border between NI and Eire. There is no end date to the agreement. The UK is in the agreement in perpetuity and cannot withdraw unilaterally. The UK can only get out of the agreement with Ireland's (and the people of NI's) agreement.
2. The UK has told the EU that the UK is leaving the EU.
3. The EU says "Tell us what you are going to do about the border to ensure there is no hard border. It isn't our problem, it is your problem because you have chosen to leave. If you don't leave, there is no problem".
4. Consider animals. EU law states that any animal crossing an EU border must be inspected unless it comes from somewhere which applies the EU laws relating to animals. So animals can cross the France-Belgium border freely.
5. Some NI farms straddle the border and animals roam freely across the border.
So what can be done?
There is no technological solution.
There can only be a solution based on NI applying the same laws as the EU does.
So NI must apply the relevant EU laws.
May wants a new trade deal to cover the border. So does the EU. But the transition period, even if extended, will end well before any trade deal is negotiated. And there is no guarantee a trade deal will be negotiated.
So, if the UK is to honour the international treaty it signed, there must be a backstop. NAd it must last until it is replaced by a Trade Deal. If no Trade Deal is negotiated, it stays.
The problem lies with the UK not understanding this.