Money. Arrowhead has done a few other (very cool) games before Helldivers 2 (HD1, Magicka) but AFAIK nothing that was popular enough to allow them to finance an expensive product like HD2 on their own. And the success proves them right: ignoring the PSN shitstorm the games is highly liked and got a lot of players. So they where able to use that money to actually build a successful product. That’s what publishers do (or one of the things): financing of projects the developer itself cannot pay for alone.