Simple: If the animals were freed they would destroy the ecosystems they were freed in (all ecosystems). They could all be killed so they don't cause any impact. The animals would be suffering from pain, illnesses and slow deaths just as nature intended. Animals would not turn vegans. The world would probably suffer a supply issue. Everyone would be weak, unhealthy and have a lower lifetime cause of their horrible diet. Everyone would be hypocrite as they kill plants and don't feel remorse just because they're killing something that can't walk and doesn't have eyes and mouth.

This is the list, add more if you wish.