It would be helpful for people not to just throw away their unsafe-for-flight things. I am talking about pocket knives, razors, scissors etc. not haz-mat substances. For example, a new Victorinox can cost ~$20, might be cheaper to courier or rent a box.
Many US airports do in fact have a "mail back" kiosk near security, where you can drop off your pocket knife, attach a form with your address and some money (postage plus a hefty markup), and have it mailed back to you. Airport Mailers is one of the companies that operates these kiosks; they claim to have kiosks in these 22 airports (elsewhere on their site they say 28).
Even if they don't, it's a matter of being a bit quick and resourceful. If you have sufficient time it should not be a problem.
Same thing happened to me at Schiphol with a rather expensive Leatherman Tool. Went back out, down to the newspaper stand and brought a boatload of postage stamps. They were even kind enough to give me a free envelope (otherwise I was going to wrap it in a sheet of paper). Dropped it in the mailbox and went back, took perhaps 15 minutes in all. Needless to say a very strange letter but was lying in the mailbox when I got back.
There are services like that in Germany. I twice forgot to check in my Victorinox knife, and they found it during the hand luggage control step. So they just offered this service and it was sent back to me to Moscow by mail. It was in Berlin in two different airports.
It worked like that: you pick an envelope from a stand nearby and put your thing there. Then you write your address on it and hand the envelope to an officer. He checks it and give you a reference number from the service company. Then after landing you go to the bank and transfer ~20 euros to the bank account with this reference number included.
Answer : co-incidence or rather obvious planning.
Suppose you were designing an area to filter physical content (things & stuff) on it's way to airplanes, in direct opposition to the intentions of potentially violent non-law abiding people.
Imagine a box directly next to your area in which at any time these potentially violent non-law abiding people could deposit substances or things which may (after a pre configured delay) instantly scatter the content of your area (things & stuff & people) all over the tarmac.
You would have to plan not only for potentially frequent rebuilding of this area (which would admittedly facilitate later improvements), but also for frequent hiring of replacement employees. This can be expensive, and might make your planning appear insufficient.
You could request that the airport move the scary box to another location in the airport. This is comparatively free.
There are additional concerns which involve other sorts of intentional hazards. There is the additional concern of increased foot traffic near what is supposedly a secured area (both user access of owner maintenance). My expectation is that either cost or coincidence is the single driving factor.
Because there are plenty of postal services you can use before you reach the airport. If you couldn't be bothered to pay attention to what you were putting in your luggage before you got to security, that's your own fault; I see no reason for an airport to expend the cost of making up for it.