Why? Not everything is free and open source. In fact, most of the operating systems used outside the "general-purpose" Linux realm are not (e.g. OS400, z/OS, z/TPF, AIX, Solaris, NonStop, QNX, vxWorks... even Windows Server).
That said, if you're really interested, HP offers free hobbyist licenses for non-commercial use of OpenVMS, with access to the OS and compilers and usable on real hardware or SIMH.
That said, if you're really interested, HP offers free hobbyist licenses for non-commercial use of OpenVMS, with access to the OS and compilers and usable on real hardware or SIMH.