Saw an interesting youtube comment that theorized Jiren's past. Just a theory but don't read if you want to avoid that kind of stuff:
Jiren is a former God of Destruction of an erased universe and his wish will be to have his universe restored. Perhaps somehow, the clown God was able to rescue him (or some form of him) and bypassed the "mortal connection to the kaioshin." They made a point to talk about his wish and it just feels like it's bigger than "I want to bring my family back to life" type of thing. Zeno seems oblivious to things so he may not know about this.
Thanks brah. Might explain as to why he fights and what his cause is. Seeing the way Jiren meditates, that his power isn't in training the physical like Vegeta or Goku but that, he mostly meditates or so far, no training that we saw. Its a pretty interesting point. Its actually made me want to go out and buy a Muse Headband.