A nomadic lifestyle is a lot more fun than primitive agriculture.

Agriculture means a lot of backbreaking work. More disease. An awful diet.

Anthropological estimates are that nomads get a lot more leisure time. And have longer life expectancies.

So the brave has a better lifestyle than the peasant. He enjoys more leisure, a longer life, and better food. However the peasant needs less space and can support more kids. (Heck, the kids practically support themselves once they are old enough to work...) Which means that when you have peasants against braves, after a while there are a lot more peasants and the outcome is assured.

And once the braves have been pushed around a bit, they are either dead or else trapped in a small enough space that they have to turn to agriculture. (With some exceptions. For instance the Mongols wound up ruling large numbers of peasants. Which still resulted in them being assimilated in the end...)

Cheers,
Ben