Well...that's because taking risks is, well, risky. Inherently.
In essence, all you're saying is "being cautious and avoiding situations that are likely to lead to risk of death means you're less likely to die." Which is pretty much obvious. And there's nothing we can do about it, because it's sort of part of the way the universe works.
Yeah, we could add more random "you didn't take enough risks, so you get zapped out of a clear blue sky" type dangers, but those don't actually hold much deterrent power—plus, they suck really hard.
So, yes: the rulers who last a long time are going to be the rulers who go for the safe option, rather than the fun option, every time. Forever.
Not quite true. You just have to do proper planning. Jenred ruled for ages while being one of the boldest Rulers on the continent. Admittedly, that took an insane amount of luck (Jenred at the end was utterly convinced he was divinely-backed in large part because his luck was so good), but a lot of it was proper planning and diplomacy (and the internal trust and cohesion of his realm) that let him capitalize on that luck.
I have seen way too many risk-taking realms that thought they didn't need planning and diplomacy get crushed. You don't have to do that stupidly, you know.
Also, don't overlook the power of nontraditional incentives. Some people want fame, some want money for training, some want to hob-nob, some want that hot lordling in the next region. Know the people you work with and really think outside the usual buttons.
