Because it would benefit the EU as a whole, as it would benefit the UK as well? Trying to "punish" the UK for leaving the EU is foolish and immature. The UK chose to not partake in the EU project, but certain areas of cooperation are mutually beneficial.
You propose cherry-picking and that's what the EU is not going to allow. As for benefits for both, sure, but as with Europol, there are ways to cooperate between Europol and non-EU members, the UK wanted direct excess to the database and that is a no-go for non-EU members. The UK choose not to be part of the club, which is their right to do so, but not being part has its consequences, because the UK is a non-EU member, they don't have the same safeguards for its citizens anymore as EU members do, so that's the reason that the UK can't be granted full access even though it would indeed benefit the EU as well.
So it needs to be taken case by case if letting the UK use some EU institutions and if so, to what degree can they participate and at what cost.
For many of the remaining EU states, being a member of the club is literally the only option - they don't have the economic strength, or wealth, or productive populace to make it on their own. Even with the UK being out of the EU, continued EU-UK cooperation is vital and would benefit the minnows outside of the main EU economies (France, Germany) a lot.[/quote]
Many would argue that the UK hasn't got the economic strength or wealth, or productive populace to make it on their own as well. We have no indication that global Britain will a great success, they are out since February 1st, we have seen remarkably little of global Britain and only seen the UK. What we have seen a Brexodus and Brexiles, less economic growth, hundreds of companies and thus thousands of jobs moved to EU countries etc.
But as I said, we'll see the true UK (un)importance in the global Britain rhetoric.