The way you traditionally try to make benefits safer--if you're going to choose risky investments--is to put more money aside than you think you need so that if you lose some of it, you've still got enough.
Here's the problem with defined benefit plans. Employers don't want to do that, because they're told that if they put too much money aside and if things aren't bad, if they get good experience and the surplus gets large, there is no guarantee they'll ever be able to get it back. So we put them in a system where we say, “You should all be very responsible and make sure those benefits are adequately secured, but if you over-secure them, you lose the money.”
Naturally, they find this a bit like a rope-a-dope. If they put too little aside, everybody criticizes them for exposing the members. If they put too much aside, everybody turns up and says, “Whoa, look at all that surplus. I guess that should be given away.”
That's a problem. Unless that problem gets addressed—there have been all sorts of proposals to address the problem, but nobody's done it--it's hard for the employers to do the thing that would most naturally address benefit security.