True, I'm assuming commodity hardware. I've never had the privilege of working on really high end systems, but my impression is that you buy or lease a cluster in a box that is simulating one reliable computer.
When you add all of the necessary layers of abstraction to get three computers to act as one computer, you added dozens of new points of failure. In the conventional case, yeah, you can now shut one down and people aren't impacted. But there's now a bunch of new ways it can break and take the system down and it is much harder to figure out why.