However, employers who wait for their engineers to squawk and bring competing offers are playing a dangerous and inefficient game, IMO. I've absolutely given raises when the local market put my long-time employees under market. (And sometimes the way you find out is a competing offer, but if you're hiring, you generally know the market.)
On your other point about companies being inefficient, I think we should stop worrying about whether employers or companies in aggregate are being efficient. In fact, I want them to be inefficient. It gives people more opportunities to take advantage of inefficiencies.
I suspect we have some sort of bias in which we apply fairness rules that are good for individuals in a community and apply it to "the system" which is not just a scaled up version of individuals interacting which either. We like to think big companies operate as a unified entity, but really, there's a lot of shit going on under the hood that may lead to very weird "irrational" decisions, many of which an individual could take advantage of.
Generally, I just prefer people to be more opportunistic, rather than trying to make sense out of non-sensical systems.
My point about inefficiency was a note to employers -> try not to be inefficient and risk-seeking in this particular area.