One of the issues that affects everything (including the Galileo satellite) is that many EU bodies have their membership legally limited to EU countries ONLY - often this was set up to keep out the US/China/Russia etc
The problem is no-one ever thought anyone would ever leave so there is no mechanism to allow ex-members to stay in. Of course they could change the rules but why would they - it would take years,cost s fortune in time and money and they'd have to get unanimous approval in most cases = even more internal argument and horse trading.
Far easier just say "Au-revoir" to the Brits and let them incur all the hassle and the costs - after all we brought it on ourselves........................