They're colorful, bite-size, and quick and easy. Read the FTC's warning on ultrasonic devices. Check it out here. Consumer Reporter Susan Wornick reports that by calling them baby carrots, the implication is that they are different in some way, perhaps more tender than regular carrots. These are full grown.
Some companies are up front with their marketing and say so.
They're just smaller. You could call them cute. But you can't call them baby, because they're not. There is no evidence that they work. You probably have a better chance of killing bugs by singing to them.