It might not be legal, but it isn't hard to find newer versions to download if you know where to look – z/OS 1.11 is floating around, it is from 2009, so still rather old, but a lot newer than the 1970s. Also you can find some OS/390 and MVS/ESA versions from before that (1990s vintage). If someone privately runs it for purely non-commercial purposes – I myself never have, but some people do – I think it is rather unlikely IBM will sue them.
The easiest way to get z/OS install media is to sign up for zPDT – it costs many thousands, but still a lot cheaper than what IBM charges for z/OS for an actual physical mainframe – and then you get the the ADCD media with that. The problem is, starting with z/OS 1.15, IBM began encrypting the ADCD media. As part of the installation, the media is decrypted, using the key on the hardware dongle – but the decrypted copy is watermarked with the dongle ID, meaning that IBM can trace back any leak to the individual customer responsible for it. This means people are no longer willing to publicly share ADCD media, although I've heard rumours of some people passing it around privately, only to trusted individuals. Someone could reverse-engineer the watermarking and remove it, but I'm not aware anybody has done that–I think people would still worry, what if they failed to completely understand the watermarking, and hence some of it survived?
Backward compatibility. In 1970, you buy an IBM mainframe and start writing apps to run on it. In 2023, you are still running the same code base - albeit with innumerable enhancements and fixes over the ensuing decades. On an IBM mainframe, that code base will just work. On Linux, it won’t even run - unless you buy some expensive mainframe rehosting package, which is full of gaps and limitations, and may introduce obscure bugs which the original lacked.
Reliability - most large-scale Linux systems are based on a distributed model - the app runs on a cluster containing dozens/hundreds/thousands of servers, if a single server has a hardware fault, the app just keeps on working and at worst some user might get an error which goes away if they retry. So, no point in spending $$$$ to maximise the reliability of any individual node.
By contrast, many mainframe customers have just one mainframe, and if it breaks they go down - which means the mainframe hardware has to be super-reliable, filled with redundancy, error detection/recovery, etc - and you pay $$$$ for all that redundancy.
IBM mainframes can be clustered - e.g. z/OS Parallel Sysplex - but only the largest sites do that. The maximum supported cluster size is 32 - I wonder if anyone actually runs one that big, 32 mainframes would be horrendously expensive - while Linux clusters with hundreds or thousands of nodes are quite common.
The easiest way to get z/OS install media is to sign up for zPDT – it costs many thousands, but still a lot cheaper than what IBM charges for z/OS for an actual physical mainframe – and then you get the the ADCD media with that. The problem is, starting with z/OS 1.15, IBM began encrypting the ADCD media. As part of the installation, the media is decrypted, using the key on the hardware dongle – but the decrypted copy is watermarked with the dongle ID, meaning that IBM can trace back any leak to the individual customer responsible for it. This means people are no longer willing to publicly share ADCD media, although I've heard rumours of some people passing it around privately, only to trusted individuals. Someone could reverse-engineer the watermarking and remove it, but I'm not aware anybody has done that–I think people would still worry, what if they failed to completely understand the watermarking, and hence some of it survived?