I'll bite, too. The reason the status quo allows systemic wage stagnation for existing employees is very simple. Historically, the vast majority of employees do not hop around!

Most people are not high performers and will settle for job security (or the illusion of) and sunk cost fallacy vs the opportunity of making 10-20% more money. Most people don't build extensive networks, hate interviewing, and hate the pressure and uncertainty of having to establish themselves in a new company. Plus, once you have a mortgage or kids, you don't have the time or energy to job hunt and interview, let alone the savings to cover lost income if the job transition fails.

Obviously this is a gamble for businesses, and can often turn out foolish for high-skilled and in demand roles — we've all seen many products stagnate and be destroyed by competition — but the status quo also means that corporations are literally structured — managerially, and financially — towards acquisition, so all of the data they capture to make decisions, and all of the decision makers, neglect the fact that their business is held together by the 10-30% of under appreciated, highly experienced staff.

It's essentially the exact same reason companies offer the best deals to new customers, instead of rewarding loyalty. Most of the time the gamble pays off, and it's ultimately more profitable to screw both your employees and customers!