The reality is outside of very early resume screening very little of the process is automated. Recruiting spends most of their day contacting people on sites like linked-in trying to find talent as not nearly enough shows up at our door.
My colleagues at FAANG companies tell me similar stories.
The cost of replacing an existing employee is also very high. In most domains can’t just plug that cheaper developer in, they need training and expertise building.
Outside of some very simple domains I don’t think your hypothesis holds up.