If the problem says, "exactly 72 inches tall," the answer is zero since there is zero area associated with 72.
Remember that for normally distributed random variables probability is equal to area under the curve. To find an area under the curve, the problem must either say at least, at most, less than, more than, between, less than or equal to, greater than or equal to, ... All of those statements give you an area under the curve you can shade and find.
That is not possible if you are looking for the probability that a person is 72 inches tall. We could find more than 72, less than 72, at least 72, at most 72, ... The answer when it is exactly 72 is just zero.
Hope that is clear,