Because many don't. There are casual fans that may not know the rules. Also, there are PLAYERS that don't understand the rules.
For example, there was a game quite a few years back where the Eagles and Bengals (I believe it was Cincinnati) played an overtime game that ended in a tie. Donovan McNabb at the end of overtime let the clock expire rather than try a Hail Mary. When questioned, he said he never heard of a game ending in a tie. Thought there would just be another OT period.