When the likely outcome is death, most people don't want to fight. If people were just allowed to flee to some other country, not only the bully country gets a free and repeatable pass but the neighbouring countries get some immigration crisis which lead to not-so-fun rhetorics.
The Finns were some of the most vigorous defenders in history, but if they just had the option of moving en mass to Sweden, consequence-free, they probably would. If that was to be the case every decade or so, today's Russia would be bigger than the USSR ever was and without any republic whatsoever, only Russians given that everyone else fled. They have quite the history of chomping territory; that's how they got this big to start with.
You probably understand that this wouldn't fly. That the other countries wouldn't allow it to happen anyway. But at the end of the day, you need boots on the ground to do anything about it.