1) Let me guess. He had the perfect 20g tank sitting on a shelf...
2) Because after store "mark-up" the price they would be charging for some of the t-stats would be unattractive to a casual shopper. They have a better chance of making a sale to an inexperienced shopper if that person thinks they can get a full set-up and snake for a cheaper price than what some of the high-end T-stats alone would cost. The pet stores only motive is profit...