Mainly because VFT's sold in the cubes are destined to die for a few reasons, and not always because the main target demographic is kids.
The main issue is that the instructions are usually, if not always, no where near what a VFT requires. Most brands that I have run across do not specify distilled/rain/mineral free water. And as everyone knows most tap and city water is a death sentence unless yours is super magically untouched.
Also, most care instructions make it seem like a VFT is a tropical plant. Most say to keep it warm, and to bring it in when it gets cold. I have never seen one detail how these plants go dormant and it is crutial to their survival.
I have seen instructions on death cubes ranging from "don't let the soil dry out" to "fill the cube with an inch of water for the plant to sit in." I have never heard or seen one that correctly details a good water regimen, especially considering dormant months. Most are to soggy or dry with their instructions, and everyone's specific conditions will effect what they need.
Lastly, a lot of these I see (but not all) are clumps of lots of small plants, fresh if the tissue culture boat. These are not hardened off the best, and are weak, fragile plants in some circumstances. Not the best when the plant will probably be bought by a beginer that will make mistakes.