Why are principal engineers so curmudgeonly?

I can understand that their 20+ year career has consisted of considering worst-case scenarios. I can understand that they are incentivized to value stability and consistency over moving quickly. I like privacy and security as much as the next guy but I'd estimate that for 2/3 of the PEs I've worked with their risk aversion becomes bureaucratic. Is there anything to be done about this or is that the role they play and we should just acknowledge their perspective and move on?

This seems like a broad generalization, like asking “Why do junior programmers make so many bad decisions?” Your 2/3rds estimate comes from what sample size?

Organizations in general tend to risk averseness, for usually obvious reasons. Stability, consistency, continuity, security all form part of a larger scale view of the business as a whole that perhaps seniors have internalized and juniors have not. Certainly bureaucratic friction and resistance happen, but so do poorly thought-out ideas and change for its own sake, or to relieve boredom with the day-to-day drudgery that describes most programming jobs.

Ask this again when you have 20+ years experience and have lots of stories of dumb and risky failures to tell.

The risk aversion is part of the job function. The principal engineer's review is based on the ability of the engineering teams to deliver with stability. If not, they fail. It is to their benefit to not take risks. Also, critical initiatives that the principal is part of aren't great opportunities for experimenting.