For the same reason any of these traveling jerks get into a rage when something doesn't go their way on a flight. Surely you have heard of the countless issues of some idiot losing their cool inflight, and having to be restrained until landing. Please give me ONE, just ONE good reason why anyone needs ANY type of a knife onboard of a commercial aircraft?
If that's your standard for permitting passengers to carry on an item, then there are very few items that pass that test, no?
I carried a small pen knife with me everywhere I went for many years prior to September, 2001, including in my pocket on airplanes, without incident. Did I absolutely "need" access to that pen knife during the flight? Of course not. But then again, I didn't need access to my car keys either, since I was on an airplane.
IMO, the real idiocy was Norm Mineta's declaration in the hours that followed that horrible day when he announced the war on Sharp and Pointy Objects, as if they were the issue early in the morining on September 11, 2001. That lunacy was compounded on August 10, 2006 with the War on Liquids and Gels.
Business travelers on short trips tend to not check any bags. IMO, they shouldn't be told that their choices are to leave it home or check a bag, but that won't stop the FAs and others from claiming that the aisles will be rivers of blood if noncriminals are again allowed to carry small pen knives with them.
How many FAs or other passengers were ever sliced or wounded with small pen knives onboard airliners between, say, 1930 and September 10, 2001? My guess is that the number is very small. The lessons of that horrific day are not that "passengers cannot ever be trusted with small pen knives."