Do you really think one can't obtain the essential components? I'm not saying it's as easy as buying lawn fertilizer.
Two things. First, I don't agree that one "can't" get the components. Secondly, and to be fair...that is not what you wrote. You said that I couldn't make a device with that material, which is not entirely true.
I don't think I understood what you meant (I also missed the one post, as I mentioned earlier). I completely agree it's difficult. I still say it's quite possible to get the other components.
Take a look at what wiki has to say about malicious use. There is a long history of abuse of AN, including ANFO bombs and ANNM (nitromethane). I know for a fact I can buy 20% nitromethane two-cycle engine fuel by at least the gallon if not more. The only question is the detonation. Given that the Taliban can make devices, I'm pretty confident any one of us could do so if we wanted to (obviously we would not...just a little shout out to anyone monitoring social media out there!). My point is simple here: Lawn fertilizer is a major component of ANFO bombs. Why is it not more tightly regulated?
I understand your skepticism, and I may not be able to convince you otherwise, but I can assure you that the required components really are beyond the reach of the vast majority of the population, which is enough to make the scenario very unlikely. Permitted users are licensed and there are strict storage security requirements that are mostly followed. The main reason for the proliferation of IEDs in conflict regions is the ready access to captured and lost military munitions that provides an easy source of those components. That is not the case in the US.
The wiki article is mostly accurate, but incomplete. Note that the terrorist groups that have used ANFO and variants already had supplies of commercial or military secondary explosives and detonators. The IRA, for example, was supplied with a large quantity of Semtex and detonators by the Libyans. Their use of ANFO was to permit them to make very large vehicle bombs without depleting their limited supplies of high-performance explosives, using those only for initiation. Even then they had mixed results, due to a lack of technical expertise leading to inadequate boost and/or poor coupling between charges. As I said earlier - even access to those other components does not ensure success without a reasonably detailed knowledge of how to construct the system.
I do not know for certain, but I would guess that the main reason that fertilizer is readily available is that it needs to be for its primary purpose, and that controlling the other necessary components, which renders it useless on its own, is regarded as adequate. It has largely worked so far, with one notable exception in which the perpetrator went to extraordinary lengths to succeed.
Contrary to the image portrayed in popular fiction, improvised explosives are really a difficult and unsuitable tool for this kind of thing.