Because experience has shown that no matter how good the EU might be, stupid politicians arrange referendums, and stupid people then vote to leave.Not what you want, but respect the democratic decision of a sovereign nation to leave the EU, without being vindictive and vengeful.
If the EU is so good( which it obviously isn't), why do they need to try to frighten other members not to leave, by punishing the UK?
Something like the Japanese deal, is well achievable.