First of all, Jenkins was created in 2005, when REST was barely past the stage of being a gleam in Roy Fielding's eye. Yet, almost from the start, almost every page had an API (you'd add /api after the URL for its corresponding HTML page). With XML, JSON and Python (!) formats, with built-in search and filtering.
Secondly, to your point, there you go: https://stackoverflow.com/questions/17716242/creating-user-i... I'm pretty sure that the reason there is no REST API for it is because you're supposed to be using your favorite back-end (LDAP, AD, etc.), with which Jenkins can integrate.
It's quite disingenuous to complain about the Jenkins APIs, there are a lot of them. They're not perfect or designed necessarily like something you'd design in 2020, but they are there.
