I think he doesn't leave them because he likes being abused. It turns him on
That is pretty fucked up
LMFAO
It looks like he might actually be in love with the seme and he just doesn't value himself at all. It seems like he'll take anyone who will have him, no matter how badly they treat him. It's also possible he doesn't want to be treated too well because then it won't hurt as much when the person leaves.
And actually you say he just doesn't learn, but your brain tries to maintain equilibrium, even if it's bad. It's incredibly hard to escape an abuse cycle because you not only unconsciously look for that because it's what your brain is used to, but predators instinctively know you're a good victim.
That is very tragic to be at the receiving end.
I really dislike victim shaming and blaming because it always takes away the responsibility of the real perpetrator.
But this Uke actually never learns, does he? He is attracted to shitty scumbags and it is the dark ML/Seme who always has to clean up his mess/save him...
I can't really relate to this character because it was myself who stopped my own destructive behaviour and my own determination to avoid more toxic men.