This may be a silly question but I was wondering why the wax layer/coating that we put on doesn't melt off in the summertime when the temps are high, sun is shining, and the surface of the car is scorching?

Point in fact is the wax does melt. That's why it loses half of its effectiveness after one month. Most waxes also blend silicon oils for increased shine, which tends to attract dust. Overall it doesn't seem like a very effective solution. Zaino lasts longer than traditional waxes but requires multiple coats.
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Because the coating is too thin to run. You can put a coating of a liquid like kerosene on a car - it will evaporate but won't run off. Try putting a wax-thin coat of oil, it simply won't run off since there isn't enough to produce the surface tension necessary to drip.

Capillary action means it won't run but it will melt. Imagine that thin layer of wax. When it gets warm it melts and dirt sticks to it. The dirt absorbs some of that wax and takes it away when the dirt is washed off.
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no wonder the dirt around my driveway always looks shiny... :)

all waxes have different hardnesses and different melting points. Both of these physical properties affect the lifetime of the wax........