1) The "leashes" are too long
First and foremost, a leash implies that you're using something to "control" something else. These suckers can extend up to 26 feet! How much control do you actually have when your dog is half way down the block and decides to take off into the street after that squirrel? That's a situation that has and can quickly turn deadly...there's a reason the term short leash exists - control. Without it, you're just asking for trouble.
2) The leash is a thin dangerous cord
About 90% of these "leashes" are comprised of cord. The cord is so thin that you could essentially use them to cut butter in the kitchen. And if you're caught in the wrong situation where the dog wraps around your leg and takes off...you're going to be the butter. It's not pretty and it happens ALL the time!
3) They're cheap and they break often
"But wait they're like $50...how can they be cheap?!" Do a quick google on retractable leashes breaking...you'll find out quickly how often it happens and the horrors of an emancipated dog running into a highway. Pet store employees will even tell you, they always see these the locking mechanisms inside these break. If the locking mechanism breaks and you're dogs going full speed at that 26 foot length he's going to either tear it out of your hand...or he's going to snap it...either way, most likely he's out free and, to his terror, there's a loud leash monster hot on his tail. This doesn't end well for anyone.
4) They're a pain to hold on to
We're athletes...or we think we are...we used to be at least! We run. We hike. We walk outdoors. We have our dog at our side every day doing as much as we possibly can with them. Holding a leash handle (or putting it around your wrist) is one thing...holding onto a piece of plastic is another. No one wants to hold on to that thing all day...it's a horrible every day leash and for those of you who take your pooch with you everywhere, let alone running, it's not an option.
5) They condition a pulling behavior
The point of having a dog on a leash is for control as we specified above. As every dog expert will tell you, a lot can be said about how you walk your dog (or vice versa). Pulling, jerking, or even walking too far in front of you are behaviors that say you're not in control...the dog is in control...and the dog is walking you. For most dogs the length of the leash alone is enough of a no-no for this one...the dog at 15 feet ahead of you is pretty much in control of his own destiny...you're an afterthought at that point. Not to mention, by pulling and allowing the dog to go further out on the leash, you're conditioning him that it's ok to pull because he'll be rewarded with more slack. As we all know, this is the opposite of what you want to condition him.