They wont let them leave for the same reason Lincoln would not let The South leave ‘the Union’.
Because who will pay all the bills?
The people who had all the industry, that’s who. “Barely a yard of cloth or a pair of shoes can you make”, as one general rightly observed.
But let half the nation walk away, good luck holding the OTHER half together when the next big disagreement comes around. And even better luck keeping out the European powers who would have loved another shot at land-grabs on the continent - or in England’s case at revenge on a nation she STILL views as runaway servants and upstarts.