Admittedly, I don’t know much about Brexit, but from what I have been exposed to, it seems like a decisively economical and political impairment that made travel and business with the rest of Europe more difficult and costly. Since it is so highly criticized as a terrible move, why doesn’t the UK just rejoin the EU?
Others have already mentioned some of the many reasons why.
But i would like to add that even if they’d rejoin, they could not do so with the same conditions that they had before. Since they joined the predecessors of the EU so early, they had a number of privileges that a new joining member nowadays would not be granted. So from that side “going back to how things were before” wouldn’t be possible to begin with.