I dunno where you got this idea – from high school speech classes? There is no team of judges to enforce rules or determine veracity. A moderator can’t be expected to know every detail of what a candidate might end up lying about. And even if a moderator calls someone out on something, then they get into a squabble about semantics and what was “really” said.
It has to be up to the other debater to call out the lies, and Obama failed miserably at that task in the first debate.