Because nobody likes playing with cheaters.

Playing games should be fun. In order to ensure the best possible online multiplayer experience, Valve developed a state-of-the-art anti-cheat system that automatically detects and permanently bans any player using cheats or hacks on a VAC secured server. What this means to you as a player is that you don't have to worry about annoying cheaters getting an unfair advantage and disrupting your game.

The Valve Anti-Cheat system is only enabled on servers with the secured shield logo and only Valve has the ability to VAC ban a user. While server admins may choose to ban players from playing on their specific servers, they do not have the ability to VAC ban players. The VAC system is automated and contacting Steam Support to report cheaters isn't necessary. Valve has a zero-tolerance policy for cheating and will not lift VAC bans under any circumstances.