Because it’s a fun game. I realise this sounds like a silly answer, but it’s the only one I have. The actual moment-to-moment gameplay is exciting, the heft of melee combat and visceral nature of big guns are both pretty, and it’s just fun to knock large crowds of enemies around. Same as Vermintide, really.
Except unlike in Vermintide, we actually get a decent selection of mods without waiting for Fatshark reps to get off their asses and “approve” them. Hate the Plasma Gun sights? Change them. Don’t like item sorting in the inventory? Get different sorting. Want to see your max ammo capacity? Add that in. Honestly, I probably wouldn’t be here without the treasure trove of goodies on Nexus Mods. People have done so much work adding in missing features.