They did - that's VAC (Valve Anti Cheat). It's so worthless even the Steam developer documentation pretty much tells you to use a third party solution.It seems kind of odd to me, that Valve wouldn't make their own anti-cheat middleware solution, and offer it for free to game software developers. That way the could use it outside the Steam environment if they want, but when they bring their games to Steam it would make it easier for games to hook into native APIs instead of the hodgepodge of look alike might be compliant Windows APIs on Steam OS. A whole suite of platform agnostic APIs would be ideal, but that seems unlikely, but a few that work anywhere the developer wants, should be beneficial to all parties.
Reminder that Valve only made VAC after they -forced to do so because they tried to "you're paid in exposure" to Punkbuster, who pulled their support of Team Fortress Classic, so Valve had to make VAC to replace Punkbuster because the game started getting mauled by cheaters again and PB called Valve's bluff. Valve has only updated it just enough to get the low level script kiddies out of games but anything advanced still requires manual reporting and someone doing the ban.