(Which typically provides a Steam key).
Many sites claiming to offer free downloads bundle the game with spyware, ransomware, or miners that can steal your personal data or damage your hardware.
Buying the game ensures the developers can continue to produce new content and maintain the servers you play on. How to Get It Safely
Since Ready or Not relies on official Steam servers for matchmaking, pirated versions almost never have working multiplayer. You will be locked out of the core cooperative experience.