There were smaller acts of defiance and even a major rebellion, but they were all responded to with extreme force. The government is well-practiced in designing the games, in combination with other totalitarian subjugation methods, to destroy the will and ability to rebel.

I know it seems crazy that people put up with the practice for so long, but the ones whose children weren't selected as tributes were usually just so relieved. People were less likely to rise up in arms when they still had children to look out for.

And if the level of barbarism seems surprising just think of gladiators. Humans have watched mortal combat orchestrated by the government before, often with unwilling participants.