User blog comment:James12708/How could no one know about the Reapers/@comment-5337704-20120816092423

Mass Effect 1 explains this very well, as does Liara, her work she has come to find out something existed before protheans, but due to her young age, no one believed her, as well as she didn't have enough to go on herself.

There were a few conspiracy theorists as well, but like most humans/aliens they would rather live in comfort of not wanting to believe than to accept their world isnt as they know it.