When I was hiring on behalf of a client, the average tenure was 1-1.5 years for mid-level in Australia. I was surprised but then I started hearing common stories. The two most common ones were the following.
The market rates were going up(and inflation going up more), people would ask their employers for an increase in compensation and get denied with broken promises. They'd wait to see if those promises go anywhere and then realize it's not going to happen so decide to change employers(and their employers act shocked).
Similarly, employment conditions were deteriorating and burn out spreading. People would ask their employers for a change in conditions such as hiring more staff and get denied with broken promises. They'd wait and well, see above.
On the other side of the spectrum, there were some with over 10 years of experience at the same place and lacked the knowledge of anything outside their employer's bubble. Their manager changed or redundancies started or something triggers and now they have to look elsewhere with their "loyalty" meaning nothing except being a detriment.