Why did Putin not close the borders? Perhaps the damage would have been excessive or perhaps this is designed to send abroad anyone who might actually dissent with the Psychos Junta.
What for? Russia has a multiple greater mobilization resource than Ukraine, it's okay if some people go abroad, it's their choice.
To stop people from fleeing the country to avoid either fighting a war they aren't allowed to call a war or face 10 years in the gulag for refusing to fight in the war they can't call a war.