I think Lethal Company is popular because it is a great experience with friends, especially with mods. The horror aspect of people just going silent when dead and not knowing if you are the last alive, feeling like everything is going fine and turning a corner to find a monster that has you dead to rights, and the non-serious almost parody meta make it entertaining beyond the core gameplay.

Helldivers is also a great friends game and the bigger picture meta of the game gives a greater goal than to just complete the mission. It feels like you are part of something bigger than just that match while you are just ripping through enemies. It apparently was originally a Halo ODST Helljumper game pitch that Microsoft didn't think was good, hilarious that they didn't greenlight it in retrospect.