Really, because I'd love to buy them. I'd have believed you if you said VMS clusters or NonStop systems. They're not x86 either, though, although a VMS port is underway.
Mainframes are very impressive beasts. The shops I know that use them needed them to do "big data" before such a term existed. They were the only game in town in the 80s and 90s to perform operations on massive quantities of data, and run massively parallel operations. Most people worry and generate alerts when their CPU goes over 80 or 90% utilization, but on the mainframe running at 100% utilization is the ideal scenario to maximize your investment.
Once you're on the mainframe though, it's a very long and difficult process to get off. And usually there is little to no business incentive to do so. Want to embark on a 18 month 7-8 figure project to migrate off the mainframe, or spend that money to build X feature our customers have been demanding? Tough sell for the CIO..