Think of the EU as a club where members don’t trust each other.
So this EU club set up certain rules that all members contributed to draft and agreed upon: if the rules are breached by one or more members, the other members can rely to an agreed juidicial body that will keep things in order.
One day, one of its most active, rich and powerful members decided that he has enough and wants to leave the club.
If the EU club still wants to exists, it must keep the sets of rules that all of its members previously agreed upon, even if one member, no matter how rich or powerful, decides to leave the club.
If the rules would be changed for the benefit of this leaving member, allowing the benefits of the club without the same responsibilities of the other members, the EU club would quickly unravel, as nobody would believe in the authority of this EU club anymore, especially when difficult / unpopular decisions must be taken for the common benefit of all its members.
When the UK decided to leave the EU club, the EU club told the UK that it can leave according to the rules that the UK contributed to draft, and the UK can then negotiate how to deal with the EU club as third party, after leaving the club.
There are already a few Countries that have deep and special relationships with the EU club without being a member (Norway, Switzerland, Canada, Japan, etc.).
The UK originally told the EU club that they want a new kind of “special” relationship with the EU club, but until last week, the UK was not even able to put in writing what they want from the EU club, once they are out.
Even now, their idea explained in more than 100 pages is not much more than a smoky wishful thinking mixed with some proposals relying on (a) overly complicated and burdensome legal solutions that have, admittedly, never been tested before; and/or (b) sci-fi technological advancements that, admittedly, do not exist and will not exist for the foreseeable future.
The EU club is not fighting hard to stop the UK to leave the EU club: most, if not all, of the EU club members resigned to the fact that the UK wants to leave and are waiting for the UK to propose their solution for the future relationship (do they want to be like Norway? It works pretty nicely for them and even during the EU Referendum campaign, Norway was heralded as a model by the most fervent Leave campaigners; do they prefer a Swiss solution? Do they want to be like Canada? So be it… ), bearing in mind that rules cannot be bent allowing the UK to get the benefit of the EU club, without sharing the same responsibilities of its members.