Complex systems, particularly those involving human beings, have a tendency to behave as if they have a will of their own that's independent from their participants. This is because they are both combining the actions of their participants and influencing those actions according to a set of rules and incentives.
Because systems such as markets and societies are so complex, and because the mechanisms of their operation are not open to observation, they often appear to behave as if they have a will of their own separate from the wills of their participants. This often leads people to mistakenly think that, therefore, they must be actually controlled by a will we cannot directly observe, that will's belonging to some shadowy conspiracy. This is no different than ascribing the complex behavior of the universe to the will of some god or gods. And, like religion, since the notion of things being controlled by a human-like will so strongly appeals to our intuition, and because it is not a result of reasoning about direct evidence, conspiracy theories tend to be as impervious to evidence and reason as religions are.
The best defense against conspiracy theories is to develop an understanding of how rules and incentives affect systems. There is a branch of economics called public choice theory that addresses exactly these issues. I think it would be hard for anyone well-versed in public choice theory to believe the vast majority of conspiracies out there.
This is not to say that conspiracies don't exist, of course. There have been plenty throughout history, and I'm sure there are plenty going on right now. But the vast, shadowy conspiracies postulated by conspiracy theorists almost certainly don't exist. Instead, there are just bad incentives that nobody has any incentive to fix.
Frédéric Bastiat's pamphlet The Law does an excellent job of laying out many of these tendencies that underly all redistributive, democratic government. One of the big ones is that once the fruits of people's labor are available for redistribution, various groups will start spending a lot of resources trying to both keep the fruits of their own labor and get more of others'. Economists call this rent-seeking. Most of us call it "lobbying". Not all rent-seeking is lobbying and not all lobbying is rent-seeking, but most lobbying is rent-seeking and most rent-seeking involves lobbying.
Once one understands how incentives work, it's no longer necessary to postulate shadowy conspiracies to explain bad outcomes from systems that seem to be designed with the best of intentions. Designing political, economic, and social systems is hard. All one can really do is to keep them as simple and transparent as possible, because complexity and opacity provide all kinds of cover for bad incentives and rent-seeking.
