[Kalimac]

The thing about Shelob is that she doesn't really move around that much - she's a real menace but if you stay away from her lair, it's not like she'll come out looking for you or sneaking in to steal people in their sleep, a la Grendel in "Beowulf." (Of course when she was young she was probably a bit more active, but we're working on the assumption that she's pretty much permanently established at Cirith Ungol by now).

This fact by itself probably wouldn't encourage the men of Gondor to go and smoke her out, so to speak; after all, if you live a good neighborhood, and there's a really bad neighborhood located a pretty fair distance from you, do you decide to to start a difficult and very dangerous crusade to clean up the bad neighborhood, or do you simply avoid going there? Someone who was very, very altruistic would probably feel they should fix up the bad neighborhood even though it did not benefit at all, but 99% of the population would probably just say "Don't go to that neighborhood and you'll be fine." They probably wouldn't see any reason to go fight and kill her, risking and losing lives in the process, when all she was doing was hiding out in an area almost nobody went to, guarding a land that nobody cared about visiting. Easier just to let it lie, they would think. She'll probably just die of old age one of these days.

This attitude would probably be encouraged by the fact that she probably moved in when there were some other fairly pressing troubles going on for Gondor; when creatures like Shelob make a run for the hills like that, there has to be something bad going on. Gondor would have had its hands full battling off the major threat and afterwards been too battle-weary to care about the minor one which had been established. Probably they didn't even talk about "the spider" - after a few generations, just stayed away from the place and told their children not to go there, rather in the sense of parents warning their children away from the Old Johnson House or the dangerous local dump. After a while, as it says in the LOTR prologue "history became legend" and people like Faramir stayed away from the place because - well, it was a place to stay away from. And on the principle of people who don't know if they believe in ghosts but wouldn't care to spend a night in a haunted house, neither he nor the others would have much desire to go up there to see whether their parents' warnings were actually true or not.

So to conclude - sorry this has been so windy - probably killing Shelob would have been the best thing to do, but it's easy to see how they could have just decided that discretion was the better part of valor and simply stayed away from her. After all, how were they to know that she'd live thousands of years?