You can just watch or read the mainstream news for a few days and see for yourself, really. They may not outright lie that much, but they constantly lie by omission - all of them. And when they are called out, it's like screaming into the void since they just ignore it and carry on their merry way. Hence, very little if any accountability, and usually because the people that rely on these sources aren't the ones calling them out, and so refuse to believe they were lied to since it challenges what they think they know.