To answer the question, that's simple: A lot of them are evidence-based, logically consistent, have historical precedent, and are based on the fundamentally correct notion that the people in power should not be trusted. That is not to say that they are all correct, or even that a significant percent are correct. Merely that questioning authority is always a wise move. Question that too, question everything.

Because as long as there has been an upper class of people in power, there have been conspiracies. You can literally read through a list of declassified conspiracies from credible sources. Operation Condor, MK-ULTRA, COINTELPRO. Most countries literally have a federal agency that's entire function is conspiracies.

Reevaluate your assumptions, take the time to go through what you believe and why. I do not deem "that seems true" to be sufficient. To me, theories have evidence. Better evidence than "planes make funny clouds" or "Of course ten foot tall reptiles wearing rubber human suits have infiltrated the upper crust of human society, have you seen Mark Zuckerberg?"