Because aquariums are open topped and do not hold heat or humidity well, it escapes through the top and class is not a good insulator. Many people do use them, often they need to be modified to work well, especially when you live in a dry environment and are keeping something that requires high humidity. Often multiple heat sources are required. For some, as you say, yourself, they work fine. But in my opinion fish tanks are for fish, there are so many options for reptile specific caging using an aquarium is not necessary.