I'm pretty sure it just comes down to bean-counting: "we have a new fulltime permanent asset for $100k" vs "we have a new fulltime permanent asset for $120k" is effectively the same thing, and there's a clear "spend money, acquire person" transaction going on. Meanwhile, "we spent $20k on an asset we already have" is.. a hard sell. What are you buying with that $20k exactly? 20% more hours? 20% more output? No? Then why are we spending the money?
It's certainly possible to dance around it talking about reducing risk ("there's a risk this person leaves, which will cause...") but it's bogged down in hypotheticals and kinda a hard sell. Sometimes I wonder if it wouldn't be easier to just fire staff for a week then re-hire them at a new salary.