Because the people who own everything want to keep owning everything and because they own everything they have the resources to propagandize the idea that them owning all the things is actually good.
I think a good question worth asking yourself when it comes to evaluating claims made by someone is “Would they still have a reason to say this if it weren’t true?” Would rich people, the politicians whose campaigns were backed by rich people, and media outlets owned by rich people have a reason to support the system which makes them rich even if their claims about it being good for everyone else were not true? Yes.