Because he knows that the trick to staying alive is to remain calm. Bruce panicked and got killed because he wasn’t saying what del rio needed to hear. Marty tells people what they need to hear; del rio, helen, the snells, the langmores etc. A great skill to have if you ask me. https://twitter.com/taofeek__/status/1254255173582704640
Spoiler alert (sorry guys!!!): he could/should have left Wendy immediately he found out. But he knew that he couldn’t execute the real plan of moving to the Ozark without Wendy. Wendy is smart and dedicated and just like him, is able to work very well under immense pressure.
Wendy walked into the fire, stood in front of bullets not for Marty but for herself and the kids. The perfect partner.