Not my thoughts, but if you speak to people on the brexit side that is often the answer you get rather than buyers remorse. I was attempting to explain why the premise in the OP was wrong that everyone now knows it was a bad idea.
A common argument is that the negotiations and implementation were carried out largely by people who fundamentally disagreed with Brexit, or at the very least saw it as damage limitation rather than an opportunity. So from that perspective, they’d say there was never much appetite within the civil service or political establishment to approach it positively or make the most of it.
A lot of people who still support Brexit would argue that what was promised was never really implemented in the way they expected. They point to things like Theresa May immediately committing to the two year Article 50 deadline, which meant the pressure was always on the UK to reach a deal quickly while the EU could simply wait us out. They argue that weakened the UK negotiating position from the start.
You also hear people say there was a completely disproportionate focus on things like fishing rights because they were politically symbolic, while bigger structural issues around trade and regulation were never dealt with particularly well. Likewise the Irish border became such a political and diplomatic minefield that it dominated everything else and boxed negotiators into compromises many Leave voters never wanted in the first place.
You can disagree with that analysis obviously, but it is genuinely what a lot of Leave voters still think. Many of them do not see Brexit itself as the failure, they see the failure as years of political infighting, weak negotiation strategy, and the government trying to satisfy the remain side rather than committing to the leave mandate they were instructed to.