Answer
The probability $1/64,974$ means that, on average, 1 out of 64,971 hands dealt will be a straight flush, because all hands have an equal probability of being dealt, and for every 64,974 hands, one of them is a straight flush.
Work Step by Step
The probability of being dealt a straight flush being $1/64,974$ does NOT mean that if you are dealt 64,974 hands, one will be a flush. This is because the probability $1/64,974$ means that each hand dealt has a 1 out of 64,974 chance of being a straight flush. As each hand dealt is independent, the probability of a hand being a straight flush is $1/64,974$ regardless of how many hands have already been dealt, and how many of those were straight flushes.
