Well Finn’s story arc isn’t really about him leaving the first order, that’s the inciting incident that puts him on his journey, so there’s not a much need to overly explain and delve into what causes him to leave. I like it the way it is. Short, sweet, we understand his motivation, now we can go on our journey. His story is about finding a path and a purpose, not about his leaving his old path behind.

It’s like, we didn’t need to overly explain why Luke wanted to leave tatooine. By all accounts, he had a pretty good life there and just “longing for adventure” doesn’t really cut it for an analytical mind of just why he wants so badly to leave. But we don’t need a whole ten scenes explaining his his uncle is abusive and his aunt is a heavy drinker and he never got picked for the basketball team. One scene of minor exposition in the dinner scene and him staring at the twin Suns longingly. Boom. Now we can start our journey. Because Luke’s journey isn’t about him leaving tatooine. His family dies and now he has no reason to stay.

Similarly, Finn’s journey isn’t about him leaving the first order, that’s just where his journey begins, so we don’t need to overly explain it. I think we can assume he’s been uncomfortable there for some time, then one scene of him reacting to the atrocities of the first order then boom: we understand why he wants to leave. No need to drag the narrative along to better explain something that is only there to be the beginning of a character’s journey and arc.