Think of all OSs, hypervisors, mainframes and their own platform, iterations, versions, flavors, builds, dependencies etc... I personally once serviced what looked like a 2003 windows server in a DB office where it has a weird logo to it, just like it was a custom/special flavor built for DB, I beleive at the time 2003 was already EOL but they were probably paying MSFT millions to keep supporting it.