It's not accurate to say that it's "incredibly rare". Most people can expect at least some dosage adjustments, if not a few trials of different medications.
However, most people will see some improvements from any antidepressant treatment, even if it's not full remission. Getting to full remission is the challenge that requires fine-tuning.
Practically speaking, antidepressant medication shouldn't be viewed as a one-stop solution to depression treatment. It should be used as a larger treatment regimen involving therapy (ideally professional, but even self-directed with books is helpful) and lifestyle changes to identify and address the source of the problems.
As such, even a minor boost from an antidepressant can help move a person in the right direction, even if it isn't a one-stop cure for the depression. We need to collectively get past this idea that antidepressants are an all-or-nothing cure all and start viewing them as an adjunct to getting back on your feet and staying there.
¯\_(ツ)_/¯
Both my psych and GP said it generally takes at least three different medications to find one that's effective for someone.
One thing to watch out for with Prozac is diminishing sensitivity in the nether regions, that's probably the biggest reason I tried a few others and transitioned to another medication.