Because that leads to groupthink, which stifles critical thinking and leads to poor decision-making. Ideally we'd have honest debate instead of ignoring warning signs, dismissing alternative viewpoints, and failing to thoroughly evaluate risks. This leads to overconfidence which can cause blind spots leading to catastrophic failures. We can't adapt to new information or actually learn from our mistakes if our shared groupthink says oh that was bound to happen. We'd stop innovating entirely.

In terms of Slashdot groupthink, no one uses (used) Windows and Microsoft was about to fall, but when looking outside of that at computer sales vs counted Linux installs, the picture was and is still very different. The reverse happened on the server, but Nadella was able to see outside the groupthink bringing Azure to the success it enjoys today.