How to Incentivize Employee Reliability:
Every year, the employees vote for the "go-to person" of the office; the one with the most institutional knowledge, the one most likely to solve your problem and have access to old information.
The winner gets 2 weeks paid vacation which begins immediately and the rest of the office is left dealing without that go-to person and everyone learns what they should have learned how to do themselves years ago.
Optional step: fire the employee of the year after everyone learns how to live without them.