[WH40K] Why leave cybernetic replacements exposed?

In 40K when a Space Marine (usually) loses something like a hand they get it replaced with a cybernetic. However when they do that they don't cover the replacement with a power armor glove. Why? unless they're building the new hand out of something even stronger and more responsive than ceramite that's literally wired into their nervous system what's the benefit of leaving the replacement hand exposed?