Mainframes too are complex, but the advantage is that IBM controls almost the entire system. In the reign of the mainframe, everyone was locked into one vendor, but there was one vendor to call when something went wrong. IBM knew how complex its systems were and employed an army of support people to maintain the systems. In comparison, some networks fly solo. Complexity weighs heavily into the reliability issue.
Networks have hundreds of components often scattered across the country.
But the team that maintains this far-flung system is often smaller than the people who had mainframe responsibility. Somehow, people made the logical leap that if networks delivered cost savings when sharing printers then it must also be true that networks would save money when used for sharing information. This commonly held belief simply isn't true. Networks that are used to share information are much more complex than ones used to share laser printers. Plain and simple, complexity means cost.