If employees invest in training up employees (and more than just the standard HR pablum anybody who isn't a complete sociopath already know and the sociopaths spend their time trying to game all of the parts of the system) then the incentive will be to retain them else they will have to go through that training from scratch again on whomever they hire.

If you give people a place to work where they don't constantly feel like they're fighting fires, where they're not always begging for a raise, and where they're at an office designed to cater to the needs of employees rather than employers then you'd be surprised how long most workers will stay there.