Because the UK had a preferential agreement with the EU and a leadership position. It would be impossible to restore that state.

Even if there were a UK referendum where a clear majority voted to rejoin the EU as a member state, not at the top table, our return to the EU depends on the EU accepting our application. We burned a lot of bridges on exit that we need to rebuild.

And also the favourite party to win in the next election is spearheaded by the very people who orchestrated Brexit. The EU will not be receptive while it seems that a majority of people are still voting for an anti-EU party.

The sensible course for rejoining is to work towards Schengen or similar, while acting as a partner state in good faith for a decade or two.