Sony & many others have proved pretty comprehensively that brand reputation isn't really impacted by breaches, even in high profile consumer facing businesses. That trickles down to B2B: if your clients don't care, why should you.
That leaves legal risk as the only other motivating factor. If that's been effectively neutered, it doesn't make economic sense for companies to do due diligence with breaches.
As far as I'm aware, Yahoo were the last company to suffer any significant impact from the US legal system due to a breach.
But you are right, at F5's size and moneys, incentives for public disclosure are not aligned in the public's favor. Damage control, in all its meanings, has taken priority lately over transparency.
completely missed your point
Sometimes when a company engages law enforcement, law enforcement can request that they not divulge that the company knows about the problem so that forensics can begin tracking the problem.
I won't speak how often or how competent law enforcement are though, but it can happen.