I've been thinking about this. I remember it being a thing that people would jokingly say on forums, stuff like "it's one of the few Outlawz verses I never skip". So I always lived under the impression that skipping Lawz verses was a common thing. Pac was very caught up in a moralistic way of thinking, so much so that his art suffered from it. He probably knew, realized, and wouldn't have it any other way, but his insistance on lifting everyone around him actually held his art back I do belieeve. It also garnered him him a lot of his reputation as a folk hero, though, so say what you will of it. But in my estimation, featuring the Outlawz on so much of his music was to the detriment of some of it. It did however fuel the false notion that they belonged there.
So when I listen to their music, I feel the same way as you, but I even took it a step further and thought "okay, so it feels like theyre passion missing from their music, but is there really?" so I tried to "objectively" assess whether that's actually the case. So if you decide on some ways to determine passion in their voice, and then just listen to their songs, it's like... They're rapping. They're not mumbling. They're maybe not screaming like Pac would some times do, but a lot of rappers that I'd never think of as "dispassionate" also never scream their lyrics, so that's not what passion is. For all intents and purposes, they speak their lyrics as if they mean them. I don't think it's their passion or career choices that make their music boring. I just don't think they're very compelling artists. I don't think they ever were. They maybe had a cool verse here or a cool line there, but you keep spewing em out like Pac and the Outlawz did, and something's bound to stick. Pac was the only truly special among them. Fatal and Yak had a lot of potential, but they were Pac's same age and still hadn't risen much higher than beign most people's favorite 2Pac side kicks. They might have had shots at lesser solo careers than Pac, that would still put them on their own trajectory, but I doubt they would ever make it on any top 10 rappers lists, ever. Not with Pac's help and not without it. I take that all as meaning that the universe and reality is telling us they weren't superstar material. I mean, if you do some statistics on how many people there are in the world, and how many become super stars, the idea that seven dudes from the same family/neighbourhood were all gonna make it seems unrealistic. They were only in the position they were in after his death because of his compulsion to lift everybody. He was playing God lol.
And I'm not saying that's a bad thing in the end, just that it's an explanation for why most Outlawz music is boring to listen to. I'll give it to them, the Retribution album actually sounds really good. But they didn't invent that. It's just a really good emulation of a 2Pac/Death Row album. It's good because it feels like it could have featured a Pac verse on every song and been a 2Pac + Outlawz album. So in essence, it's good because it sounds like a 2Pac album, not because the Outlawz pioneered and made great art. It's not "their" sound. Who knows, maybe Pac even picked some of the beats and helped write some of the verses for it. Some of the songs were maybe even recorded before he died.