This is a complex issue, but one key component is this the relative bargaining powers of the parties [1].
Supply and demand in the job market is only one component of the bargaining power. Social norms and many other factors play in.
The field of Organizational Ecology [2] also can offer some metaphors about what is happening. Both herd behavior and organizational inertia are significant factors that explain how companies interview.
[1] https://en.wikipedia.org/wiki/Inequality_of_bargaining_power