I think it's mainly time and sometimes you don't realize how much knowledge/expertise you have because it becomes second nature so you don't realize how valuabl

I think it's mainly time and sometimes you don't realize how much knowledge/expertise you have because it becomes second nature so you don't realize how valuable sharing it would be. If this is in an office setting, sometimes co-workers won't document/share knowledge so it makes them feel like they can't be let go/replaced.