The existence of stupid terrorists is a nuisance in the grand scheme of things. The inspiration for the level of our efforts is fear of something on the scale of 9/11..or bigger.
There are people with the funding, means, and motivation to do something like deliver a fully weaponized anthrax bomb in NYC or Washington, DC. A large portion of our intelligence efforts are devoted to making sure that they do not succeed.
If those people really exist with the motivation and means to do that, why haven't they done so? US Intelligence is clearly not perfect, as we saw in Boston.
First, existence proof. Do you think that al qaeda under Osama bin Laden didn't have the money, means, and motivation to have done something like that? If you think that, you should look into the situation at the time, Osama was from one of the richest families in Saudi Arabia, had a network that extended around the world, etc, etc, etc.
Now why did he opt for planes over mass destruction? I don't have a window into his mind, but I can make some reasonable guesses.
First, he was looking for great visuals, and he got that. Killing 100k people with anthrax would have done more damage, but would not have the visuals of seeing an airplane disappear into a building and explode out the other end.
The second has to be concern for the US response. He took out high priority targets that struck at the heart of American power without undue casualties. He got multiple wars, and a worldwide manhunt for anyone in his network. If he had inflicted hundreds of thousands of casualties, do you think that the USA would have held back on all out war? (Potentially including nukes, a draft, etc.)
A third is that the USA has been proactively identifying ways in which he could actually acquire those things, and blocking them. For instance look at the efforts we took to help secure Russian nuclear materials.
The effectiveness of biological warfare lead to major world powers accumulating hundreds of tons of such weapons, considering what would happen if they used them, then signing https://en.wikipedia.org/wiki/Biological_Weapons_Convention and destroying them. (It is, admittedly, easier to give up one "ultimate weapon" when you retain your supplies of another. And in warfare there is no question that nuclear bombs are more useful than biological weapons because they act immediately.)
The capacity to create them again exists and takes little resources in theory. Doing it effectively and actually having it work the first time you try is another story. There is no question that al qaeda can find people who could try, but the probability of success is a different story.
