Because the sticky stuff usually adheres better to your snake than the tank. Besides, those analog gauges are inaccurate anyway, and you need to be measuring temps and humidity down where your snake is lying, not on the side of the tank. Get a nice digital gauge with probes.
Trying to save money is my biggest issue right now and I just spent $11 on the set. Doesn't sound like much but it is too me. Could I glue them to the glass for ambient temps?