In a high availability environment (like banking or airline reservations, as mentioned), mainframes never go off, even during upgrades. When physical machines need replacement, the entire system is run in parallel on a second machine.
These boxes have both incredible I/O hardware (there's never been anything like IBM channel I/O) and the software to keep everything humming. On a typical day when I worked there, we'd have a 1,000 devs using the same box (and that was a small installation) with multiple operating systems, virtualizations, etc. with zero hiccups.
IBM also has amazing software for getting stuff done. Documentation, for example, is universal and available worldwide from any terminal. When you want a printed manual, the system figures out which big, fast printer is near you and offers to print it down the hall. Every IBM employee has access to all the company's resources from every terminal, and it all just works.
Yes, PCs have caught up to emulating many of these features, but they definitely don't have the robustness or ease of use that System/370 (z/OS) did.
That's an aside though. The reason people keep buying them is software lock in, yes. There's a reason Amazon likes to push their proprietary services...they are the new mainframe.