Welcome said the nightman
We are programmed to receive
You can check out any time you like
But you can never leave
Why have states throughout the world, and throughout history, not allowed secession to occur within their territories? Various reasons are offered here.
States typically argue that secession is not legal. But claims to legality do not necessarily align with what is lawful or right. A group of people cannot be held in thrall to a government to which it does not consent.
A second argument is that secession destroys unity within the state. But unity is not a requirement for prosperity. In fact, diversity, discomfort, and disputes with the status quo are vital for progress. Moreover, if others cannot persuade those wanting to secede, then 'unity' becomes a construct of force rather than volition.
A third argument is that secession makes the remaining state weaker to defend. But how can a state be strongly defended when people inside its various territories do not wish to be there? Moreover, federations of states are capable of forming defensive coalitions while remaining independent.
The likely reason for government opposition to secession is that the state loses control over people and resources when some fraction of the governed decide to break away. Because it does not want to give up its 'property,' the state is likely to fight secession with maximum force.