What I don't get like you say, is we have had 3 years to sort this out and yet we are still not progressing forward with it.
Unfortunately, the people who want to leave have numerous incompatible ideas, and have been unable to agree on a course of action. To make a success of Brexit, they needed to agree the objectives before making the decision to leave. They still haven't agreed the objectives, and they say "no" to every alternative they're offered.
Before the referendum, Nigel said we'd have a free-trade agreement with the EU, and access to the single market. Now he says that Brexers didn't vote for that, they voted for no-deal.
Today, the Brexers say that the Northern Ireland border will be no problem so the Backstop we agreed (which only comes into force if no alternative solution is put in place) will never happen. However, they don't actually believe that, which is why they say UK can never agree to it.