Because the RPM of a fan is not a 100% reliable indicator of its loudness. Sometimes a lower speed can even be louder than a higher one, because of certain resonances...
Yes, that could work (it would need to be done for each fan individually, because of manufacturing variation). But it's still not guaranteed that the fan will keep working the same way in the future, after some use/deterioration or in untested conditions.
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Because the RPM of a fan is not a 100% reliable indicator of its loudness. Sometimes a lower speed can even be louder than a higher one, because of certain resonances...
Presumably, knowing the fan you've installed into the device you've designed, you could map those noisy areas...
Yes, that could work (it would need to be done for each fan individually, because of manufacturing variation). But it's still not guaranteed that the fan will keep working the same way in the future, after some use/deterioration or in untested conditions.