Not really. It's just viewed that way because our society for whatever reason craves a hyper rational, airtight legal system. That has its benefits but it's not a perfect system either.
The alternative to "I know it when I see it" is the following framework for obscenity where all 3 have to be true for something to be obscene.
- Whether "the average person, applying contemporary community standards", would find that the work, taken as a whole, appeals to the prurient interest,
- Whether the work depicts or describes, in a patently offensive way, sexual conduct or excretory functions specifically defined by applicable state law,
- Whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.
#1 and #3 are still entirely subjective and even #2 is largely subjective. Something that is clear obscenity according to #1 and #2, but a judge finds has some redeeming "artistic merit", is suddenly not obscene. That is no more airtight or rational than Stewart's standard,.
Plus, judges are put in their positions to...make judgements...often in murky waters. It's kind of what they're best at.