Edibles tend to be popular with inexperienced users who don't want to smoke, it's impossible to know exactly what dose you're eating, and you won't feel the effect until it's too late. Ergo, it's very easy to overdose with them.
One company called Spot (in Seattle) has a little chart on the back, as well as these informative numbers, that roughly equates the dose of a cookie to a certain number of glasses of wine for further expectation setting. Overall, the packaging is very clear[0] about dosage, the time of the onset, and other factors that may contribute to the type of buzz you'll get.
It's very much similar to ABV on an alcoholic beverage, though. You can know exactly what you're ingesting, and you can get the general idea of what to expect, but it's impossible to label a can of beer with something like "5 of these will make you drunk in about an hour and half".
--
0. http://twotentwice.com/wp-content/uploads/2015/01/SPOT-Bites...