If you're not breaking existing laws, why create others to make sure you're less likely to? If you are breaking laws, well - There are already laws in place to enforce that.
Existing laws against reckless driving are full of vague generalities... What exactly constitutes driving with "due care and attention"? As your burrito example indicates, there's a continuum, and reasonable people can reasonably disagree on what's legal and illegal under such statures. Those sort of laws are best reserved for the extreme cases that legislators never could have imagined up front (If that story had been real and if the guy had lived to see a courtroom). New, more targeted laws can set consistent expectations about what's legit and what's not, so that drivers, police, and judges are all on the same page. That's the opposite of "more complicated"... it's simpler because it clarifies what is and is not considered a problem for an extremely frequent set of opportunities (cell phone & texting usage).
(And no, they aren't going to pass a similar law for burrito assembly, because that's extremely uncommon. Sometimes the law is a little bit like code optimization: just as you target the 5% of your code that's taking the bulk of CPU time, so do legislators target specific behaviors that are especially vexing to their constituency/campaign contributors.)
It seems that the only way to enforce "distracted driving" is if the driver is doing something else wrong.
Enforcement is difficult... I know a city jurisdiction that only caught one violator of its anti-texting law in the first year of operation, and it was only because the man admitted his usage to the cop. However, even with minimal enforcement, the mere presence of the law can subtly encourage the right behaviors: humans have a surprisingly strong instinct to follow the rules, especially when this instinct has been cultivated in their upbringing.