Some of the worst press bias goes under most people’s radar for the simple reason that it’s accomplished by omission. To spot an omission, you have to be aware of what is omitted, and that can only come from knowledge gleaned from other news sources (or in some cases, from your own experience).
But if people are only reading the MSM, and tweeting in an echo chamber, how would they ever learn what’s missing? It’s less of a problem on the right, for the simple reason that since so much of the MSM skews to the left, it’s harder for the right to avoid what the left is covering. However, it’s very easy for people on the left to avoid stories on the right.