I mean this isn't unprecedented. Blackwater exists in the United States, hell you even have the Pinkertons, if you go back the revolutionary war the Brits used the Hessians to fight.
Money, most of them come really poor backgrounds. Imagine living in the ghetto and getting offered well over $100,000 per year in the US.
The actual numbers I think soldiers start at 3k per month. Which is a shitton of money for somebody in rural Russia/Moldova/Ukraine. ( yes ukranians have been known to be recruited into Wagner before the war)
There are also sometimes some of the same principles of found family and inclusion that get kids into gangs too. A military unit can very much feel like a family sometimes (depending on the unit). Once the money lures them in the brain washing starts to make them a cohesive fighting unit and that often includes group loyalty and strong feelings of inclusion.