I recall reading that every IBM mainframe OS release had 1000 bugs in it. (This was in the 1970s/80s.) Sorry I can't supply a source, after 4-5 decades. It was probably an article in Datamation magazine.
Now, I don't know how many of those bugs were privilege escalations, or could be leveraged to be. I'm not sure that people tried to exploit in the same way back then (or, if they did, if they said anything publicly about the results). Still, even if the majority of those bugs were not exploitable, OSes were much smaller then.
So that's my conclusion of how we got here: We've always been here.
Mainframes went right to virtual machines, so possibly bypassed much of the multiuser faux-security which has always plagued unix. Of course, now they run Linux partitions and get all the same bugs.
The surrounding systems that connect to mainframe also expect and support those bugs. For 40+ yr now :)