It's because it's a pita trying to keep a probe in position in a tank with a snake moving it around and an accident waiting to happen, if the probe gets moved your hot spot could get too hot and burn your snakes. The probe directly on the heat tape is the safest and the best way to regulate your heat. You get the hot spot in the tub the correct temp by using a temp gun on it and then adjust the the thermostat as needed until you get the temp you want. For example, my herpstat is set to 95 with the probe directly on my heat tape and that gets me a 90 reading on my hot spot in the tub.