Because we have brains, Cheryl, and can evaluate risks and make decisions. You know, critical thinking? Questioning rules is wonderful and healthy! I teach my kids to NOT just blindly follow rules but to find out the intent behind them if they don't make sense to them and choose to follow the rule if the explanation makes sense or question further or - gasp! - break it and take the consequence if caught. That's not raising anarchists, it's raising people who can think for themselves and change what no longer makes sense (or never makes sense).
So if we're all supposed to just follow the rules because they are rules, Rosa Parks just should have moved to the back of the bus? Molested children should really just stay quiet because someone in authority told them to (yes, that's what happened in the Catholic church)? Following every darn rule out there is a slippery slope. The Facebook "rule" is arbitrary. The folks at FB didn't do congnitive-behavioral research and consult with experts and conduct studies that unequivocally determined that literally on a child's 13th birthday, something magical transpires that then makes them able to "handle" an account. It's just the age they picked. It could just have easily have been 12, or 14, or 10, or 16 and 4.5 months or whatever. It's not magical, it's not a law, it's not about safety - it's an arbitrary cut-off and is handy in case a parent wants to pull and account for a younger child. Otherwise, unless reported, the FB police aren't going to know that someone age 12 set up and account.
And really, don't worry about all of the rule breakers making it hard for you. If you believe that a rule is a sound one and worth upholding, uphold it. If you think it's stupid and arbitrary, then work to change it or go ahead and break it and risk the consequences. Personally, I will hold the line on teen driving rules and underage drinking, because I believe that the laws make sense and save lives. But if one of my kids smokes pot in college while in one place for the night (not driving anywhere of course), who cares? Not me. It's simply about critical thinking, choices and responsibility.
ETA: I take back the part about FB's age restriction being arbitrary. It's not - it's to comply with the FTC's The Children's Online Privacy Protection Act of 2000. So there you have it - it's not about safety, it's about protecting FB from having to deal with violations of a federal act that requires strict controls around who can access the information that children under age 13 post and requires verifiable parental permission for collecting personal information on a child. The act seems not worth the effort of compliance unless a website is geared specifically towards children and FB is not. Again, it's not about safety it's a CYA move by FB.